I will take the opportunity to ask:
which is best for forretress in most cases when facing the meta?
hp ghost or hp bug?
I have seen several players leaving explosion aside, before they prefer toxic or giga drain (which is not effective against cloyster), what do they think of that?
It kinda depends of the purpose of your team - unlike Cloyster, you're gonna need to address not only the value of your Forretress
as a Spiker, but
as a Spinner too (usually). Overall, i'd like to suggest these approaches:
Giga Drain Forretress: You have an awesome 1v1 match-up vs. Cloyster and Golem, at the cost of an uncomfortable match-up vs. opposing Forretress (except if you're using Giga + HP Fire) and Ghost-types (and Starmie, to a lesser extent). These issues can be fixed by pairing Forretress with a Pursuiter or / and a Spinblocker | back-up Rapid Spin user.
HP Fire Forretress: It's very good choice on teams with a Pursuiter IMO. It's quite troublesome to make a direct progress vs. Cloyster and Starmie (especially because of the reduced HP stat), but the clear advantage against opposing Forretress can make a difference between a excellent, and a terrible match-up! Keep in mind that you might find some trickier match-ups here and there (ex: non-Crunch + Pursuit Tyranitar have issues vs. Starmie, Pursuit Umbreon can't outdamage Rest Misdreavus' sleep loops, etc).
HP Ghost Forretress: I'd highly advise pairing HP Ghost Forretress with a Spinblocker - it significantly improves its performance vs. opposing Ghost-types, and having a Ghost-type of your own ensures that you won't need to play an undesirable stalemate / uphill battle against opposing Forretress.
Forretress + another Rapid Spin user: This is one of the few combinations where Forretress finds a lot of room to adjust its moveset (you can choose to use it with or without Rapid Spin - both are fine, by the way). Between Explosion, Giga, Hidden Power Fire / Bug / Ghost / Bug, and Toxic, you have a plethora of viable options here. I've seen people running specific moves such as Protect or Rest, and I think all of these moves are quite alright too, depending of the purpose of your team.
HP Bug Forretress: Personally, I've been having some trouble to find the optimal set-up for HP Bug Forretress. Starmie aside, I think this variant of Forretress can't address
all of its obstacles reliably, namely: Spinblockers, Cloyster, and opposing Forretress. The only approach that I've been enjoying with this variant, was pairing a HP Bug Forretress with a Pursuiter AND a Spinblocker, or pairing it with another Spinner. Hitting Jynx, Exeggutor and Espeon for SE damage is awesome, though. I'd like to know what other people think about HP Bug, I think it has some hidden potential in the current metagame...