Oo, great choices all around! I'm not sure I'd want to be Grimer/Muk though, or even Bellsprout. Despite all being some of my favourites, they have a huge problem with mobility! They can't even jump D: I think I'd want to be a Scollipede. Think of the sheer power and stature you'd have! Also if you wanted to get around quick, just use Steam Roller :D
New topic: 'Which Poison type move is your favourite and why?'
For me, it's a cross between four moves. Poison Jab, Sludge Bomb, Toxic and Acid Spray. Poison Jab and Sludge Bomb are both very powerful moves often causing great STAB damage and even with a chance to cause poisoning; these seem good in my books! Then there's toxic. An utterly unique move supplying so many useful strategies and coming in useful in almost every situation. It's a shame it doesn't work against Steel types! Then the other move, Acid Spray, is possible the most useful of all. With only 30 base power it isn't exactly a great damaging move, but it sure has a useful secondary effect! Always lowering the foe's special defense by two whole stages whilst also causing damage is invaluable. It means you can really cause some devastation, even with some pretty weak attacks!