they're all very intriguing Pokemon to me. Thinking about how they move to execute Tackle attacks, how they eat if you don't evolve them, and just the general idea that what's a mere transition phase in real life can be made permanent if you so please. For those who've played/intend to play Mysterious Dungeon, I suggest just walking around town as a Cocoon. Such fun ^^.
Plus, weak Pokemon in general seem to interest me, because with enough leveling, even they can slaughter several Pokemon with little effort whatsoever (even if they're low-leveled wild ones), and I always find such a sight interesting to picture in a more realistic setting. It's also funny to think of that poor sap of an early in-game trainer suffering the humiliation of his/her entire team being wiped out by a pupa, a worm, a Koiking, a Himanuts, etc. XD
But, as for which pupa is most interesting to me.... don't know. Sanagiras is out since it's actually a fairly powerful Pokemon and not as interesting as the weaker ones. Karasalis and Mayuld are pretty wierd in that the programmers gave them the exact same stats in the games. Plus, the games portray them as spikey, while the anime portrays them to look more smooth, so I can't help but wonder which form's official. Nontheless, they seem like good Pokemon, and probably the most capable of the (bug-type) pupas, with higher attack and having both Tackle and Poison Sting, plus they look pretty cool. I don't think I'd vote for them in the end, though.
Cocoon's pretty interesting, since working with only Poison Sting sounds like quite a challenge. However, it does get STAB, and it has a touch more attack power than Trancel (not to mention added chance of inflicting poison upon the enemy), so Trancel could be even more challenging to raise... though, I also gotta pity Cocoon, since it's thus far the only pupa that wasn't portrayed as mobile in the anime at this point. Trancel, Karasalis, Mayuld, they all got to hop around and use Tackle during at least one point in the anime, but poor Cocoon, all I've seen it doing is hanging in trees, waiting to evolve into its overly-used final form. I think I'll go with Cocoon in the end.
EDIT: Let me end this post by saying that to all the people voting for Karasalis... are you only voting for it 'cause it evolves into Agehunt? As you can see, I like all the pupas for what they are, not for what they evolve into. Isn't there anything you guys like about Karasalis itself?