Of these, I would say Forretress is the most reliable, as it's Sturdy ability pretty much guarantees at least one turn of set up. It is also immune to toxic spikes, learns the always useful Rapid Spin, and has quite the impressive defense stat. It can even take special hits with investment. However, Smeargle can also be used, as it has any move (except chatter) at it's disposal, allowing it to put foes to sleep with spore, or bring in a teammate safely and healthily with Lunar Dance. Omastar may be used, but it is much better as a Special Shell Smash sweeper. It should also be noted that Omaster must be bred the moves spikes and toxic spikes from a Cloyster that knows both, and then tutored/tm'd Stealth rock, while Forretress learns both spikes and toxic spikes by level up and be tm'd or tutored stealth rock. Smeargle must sketch it's moves, so you must either find an opponent with said moves.
Hope this helps