I know I am late to the sun discussion, but I had church camp and I have no regrets.
To start out my opinion on this whole suspect test, I am going to go through the basic fundamentals of this suspect, and break down my thought process and reasoning for my main entree of the entire post. First of all, the main goal of this suspect is
to make sun unbroken and healthy. While keeping collateral damage at a minimum is important, it isn't our first priority. This means we shouldn't be throwing out any of these options right away. The best way to go at this suspect is to start with the weakest ban option (the option that will effect sun the least), and see if it is enough to make sun healthy and unbroken. Keeping this in mind, this is, in my opinion, the order of bans from weakest to strongest in terms of lowering Sun's effectiveness.
Do Not Ban- Obviously, Not banning anything is the weakest option to weaken sun, because you aren't weakening anything at all.
Ban Heat Rock- Heat rock and Victreebel are, in my opinion, very tricky to order because they are both nerfing Sun by about the same amount. 3 turns less of sun is huge to the playstyle, because it decreases the efficiency of one successful Sunny Day, and really requires you to use your sun turns to the fullest. With 5 turns of Sun, you will probably only end up with 3 for your sweeper by the time you get them in safely. This means that your Victreebel, Sawsbuck, and Leafeon are not going to sweep an entire team with one Sunny Day, and they are much easier to deal with. This would also make it very important for sun setters to stay alive and healthy to get numerous sunny day's off. This ban affects sun as a whole for sure.
Ban Victreebel- Victreebel IS the face of sun. Without Victreebel, I guarantee hardly anyone would be using sun as much as they are now. This doesn't mean that sun without victreebel is unviable though, because other people in this thread has posted experiences with other sweepers and they are still looking very usable. Banning Victreebel is in my opinion more drastic than heat rock because I think that sun teams without heat rock are still better than sun teams without victreebel, which is the bottom line in ordering this list.
Ban Chlorophyll- Chlorophyll is more drastic than Victreebel because without this ability on it, Victreebel is not viable as a sun sweeper. It is simply not fast enough to clean up everything, and it will get ko'd before it does it's work. Not only does it make Victreebel useless in sun, but it makes nearly all other chlorophyll sweepers useless in the sun too. The combination of these two things make it more drastic than simply taking victreebel out of the picture.
Ban Sunny Day- This completely takes sun out of the picture, which is clearly the most drastic ban.
now that I had a good idea of what i should look at from first to last, I started testing each ban in this order. After drawing up enough data on every route we could go with this suspect, I drew up reasonable conclusions on each ban. Now on to the main course of this entire post.
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Conclusions on each ban option
Do not ban
For me, this was the easiest option to discard. Very little testing was needed to be clear that Sun is too much of a problem to not ban anything. My first reaction was that Victreebel is the clear culprit of this entire suspect even happening. Its access to chlorophyll, weather ball, stabs in sludge bomb and solar beam, sleep powder, and it's setter's access to heat rock allowed it to have the tools to effectively wreak havoc and implant nightmares on most teams it came across. Very few things can handle well built sun teams with standard sun victreebel on it. This threw out the idea of not banning anything completely for me.
Ban Heat rock
This little rock does a lot for sun teams, and it was apparent to the start for me that banning it would definitely nerf sun a fair amount. The question I had to ask was, is it enough? Before i left to camp, i was sold on the fact that no heat rock was enough to definitely make all sun sweepers perfectly fine in PU. I knew that with only 5 turns of sun, the sun sweeper would generally only have 3-4 turns to sweep or dent the opposing team for each sunny day. The only question was if Victreebel could still do enough damage in those turns to make it broken and unhealthy. I tested no heat rock sun setters with the standard looking sun build of Onix/Volbeat/Zebstrika/Victreebel/Leafeon/Rapidash, and I saw a big drop in effectiveness in the sun team as a whole. I would have to use my turns i had as effectively as i could while conserving my setters more, and it still was sometimes just not enough to break through many teams. Thinking through it all, I came to the conclusion that it is enough, until I read through this discussion today after getting back from church camp.
I never thought of the idea of having 2 or more durable sun setters+victreebel rather than onix+volbeat and the usual sun skeleton i seem to see on the ladder and in this thread, and this was put in my head after reading some of WhiteDmist's thoughts. In theory, having 2 or more durable sun setters with victreebel would make victreebel have the same effect it did on the teams using heat rock, and end up breaking through teams like it did to begin with. The only difference is that you have to play very carefully to get victreebel in safely and let it use its turns of sun effective, while still keeping your sun setters alive as long as possible. This style is harder to pull off than the standard sun, but I could definitely see it work out. Therefore, since banning heat rock could just lead to teams with 3 sun setters and Victreebel, I think banning heat rock isn't enough. Victreebel will have the same effectiveness if used in the right teams and played properly.
Of course, I haven't had time to test those types of teams out yet, and I hopefully will be able to in this last 48 hours. But in theory this looks very accurate and leads me to go to the next option.
Ban Victreebel
After all of my thinking, testing, and reading from before and after church camp, I think this is the option we need to resort to. The options I discussed above so far are not enough to make sun healthy again, and this one is just enough to do the job. The main question we need to ask when seeing if this ban does the job is, are there any more chlorophyll sweepers that can sweep like victreebel can to make sun unhealthy or broken? After seeing other people test pokemon like tangela in place of victreebel, I think there is no other pokemon that can sweep like victreebel can. Tangela is the closest at doing what victreebel does, and it doesn't have poison stab to make sludge bomb as strong, and it doesn't have weather ball to make a strong fire attack either like victreebel. It just doesn't fit the bill at creating enough damage output that victreebel does to break through teams.
The other question we must ask is, are there any other factors of sun that is broken to merit any other drastic options? The answer to this question is also a no. Without heat rock, Victreebel still can do what it does best. the fire attackers are also walled much easier than victreebel is so they aren't making sun unhealthy. the sun setters themselves aren't broken by any means, so the only thing that makes sun broken is the chlorophyll sweeper offense, namely victreebel. Without Victreebel Sun isn't broken to me, and that is all we need to get the right answer.
Banning Victreebel is the least drastic, and still an effective enough way to make sun healthy and unbroken. Yes, banning Victreebel means that some of it's other unbroken sets won't get to be used. However, like many other people in this thread have stated, we have done things like this in the past in smogon, and our main priority is making sun healthy and unbroken, not keeping victreebel's other sets allowed in PU. This is the answer that has the least amount of collateral damage while still doing the job.
The Other Options
The other options we have don't need to be considered with the logic that i used to come up with an answer in suspect testing. the other options are more drastic with more collateral damage, and if we already have a less drastic answer that still gets the same job done, that is what we need to do. Banning Victreebel, Chlorophyll, or Sunny Day all three make Sunny day unbroken, (or not allowed to begin with) but banning victreebel simply is the least drastic one and most efficient one to choose. Banning chlorophyll hurts other sun sweepers that don't need to be hurt, and banning sunny day takes it out altogether, which is not the problem. The problem is victreebel's strong offense, and heat rock doesn't do enough to stop that. These other options are options that will work, but shouldn't be chosen unless we find a reason why banning victreebel isn't enough to do the job.
This is my complete opinion, thought process, and stance on this suspect test. I'm sorry it is such a wall of a post, but I wanted to get all my thoughts put out before the discussion ended. I will try to test more on using Victreebel+3 sun setters with no heat rock, but as of right now I feel very confident that heat rock simply isn't enough to do the job.