i actually really enjoy playing as tracer despite her low damage output since you actually end up getting a ton of eliminations due to all the chip damage. tracer feels extremely high risk high reward in the way that she's possibly the ultimate poker against defense heroes and tanks, and she actually has a decent matchup against any offense hero as long as recall isn't on cooldown bar mcree if his flashbang lands. even if mei begins to freeze you, just recall out and you essentially have two lives to fight someone with. also the pulse bomb is fun and pretty much guarantees a kill as long as you don't stick a full hp tank hero :(
not including tracer, my top 5 heroes would probably be:
soldier 76 - solid all-round offense hero. doesn't require much team support since he's pretty much able to do anything; he can heal himself + teammates, he has explosives to dent fat tanks and one shot frail offense heroes, and his sprint is probably the greatest asset because he can flank easily, get to choke points faster, and it doesn't run out. his ulti also pretty much guarantees the win in 1v1s against non-tank heroes that aren't mcrees with flashbangs, and it's really fun when your team has co-ordination. one of my friends shielded me with reinhardt as I aimlocked three people to death in a matter of seconds to get the potg and secure the win
mcree - flashbang is literally the best thing in a close quarters 1v1 since the insta-stun basically sets you up for a free FtH on a stationary target. aside from that, it's basically FtH, combat roll, FtH, wait for combat roll to get off cd, rinse and repeat with a pinch of flashbangs. fun as hell, though i think flashbang should be nerfed a little bit; maybe a slightly shorter stun duration? basically a repeat of what Asek is saying; tons of fun flanking and shredding supports / defense heroes and abusing the ulti behind a reinhardt shield
roadhog - hookstun is amazing vs those pesky tracers and speedy offense heroes since you can basically rein them in for a point blank shotgun blast which usually shuts them down. add to that the insta-300hp heal and you're extremely hard to kill if given proper support. his ulti doesn't seem that useful though, but his tank status and damage potential definitely makes up for it
mei - bae. icegun basically shits on anyone close range as you either icicle them in the face for the kill or just wall them off if they're a tank. tracer can't recall or blink if she's frozen, and mcree can't use his flashbangs either. ice wall is amazing in terms of defensive utility, although you might accidentally block a teammate's ulti from hitting or make them miss by boosting them on the wall. the cryo freeze feels kinda meh since half the time you get ganked as soon as you thaw out, but her ulti just acts like a wide range icegun which can definitely buy you and your teammates some valuable time. amazing all-round, especially when you outsnipe a widowmaker long range
junkrat - love the aoe nades which explode on impact. there should not be a instance where you are not spamming his nade launcher, because just by shooting in the general direction they're coming from, it's basically great area denial. concussion mine is really fun to play around with too, since you can set it to explode and kill someone who keeps avoiding the direct impact hits. the steel trap is also pretty cool for watching your back. his ulti is really fun to use since it can climb walls to sneak around the opponents watching for flanks and it can potentially take out full hp tank characters