In regards to Amoonguss:
I've used this poke quite extensively in Gen V (in lower tiers, mostly), and I think that it's a lot better than people think.
Amoonguss @ Assault Vest
- EVs -
Regenerator
Giga Drain
Sludge Bomb
Hidden Power Ground / Fire / Ice
Foul Play
Synthesis and Spore aren't essential. I find that a tank set with Leftovers and Synthesis has too little offensive presence and sits there doing very little. With an Assault Vest, Amoonguss can invest in Defense and Special Attack, which gives it better mixed walling abilities and more power. Foul Play is excellent as always, and often does a better job of punishing setup sweepers than Clear Smog.
Amoonguss does face competition from Venusaur as a bulky Grass / Poison type, but its greater special bulk, access to Foul Play, and of course Regenerator give it enough to distinguish itself. I'd say that it's not outclassed, but different to MegaSaur.
Amoonguss requires very little support. It doesn't really care about status, being immune to almost every form of Sleep, immune to Toxic, and far too slow to care about Paralysis. Burn kinda hurts, but with Giga Drain and Regenerator, it hurts less so than for other Pokemon. Amoonguss is a great bulky pivot, and has somewhat low opportunity cost if used well - it has enough offensive presence to pose a threat, and can keep up with fast-paced pivot gameplay thanks to Regenerator.
Mega Venusaur eclipses it, in that it's ranked higher, but Amoonguss shares many of the traits that make MegaSaur good, which makes it a great choice for teams that already have a Mega.
I'd suggest B+ rank for Amoonguss.