I'm level 32 and considering a few mons to power up or eventually max out. I've got about 250k stardust though, so I'm open to suggestions about which ones would be actually worth it.
1) Exeggutor, level 22, 84% IV, C+SeB
2) Snorlax, level 25, 40% IV, L+BS
3) Snorlax, level 19, 27% IV, ZH+BS
4) Blastoise, level 20, 93% IV, WG+HP
5) Cloyster, level 26, 89% IV, FB+B
6) Gengar, level 22, 93% IV, SC+ShB
NOTE: as you can see I'm open to some "fun" options even though they aren't necessarily elite or must-have mons. I like them, so if they're fun to use while still effective in battle I'll be happy to spend some dust on them - for the same reasons I've already powered up Alakazam, Slowbro, Poliwrath, Tangela... (let's be honest, Blastoise is so much cooler than Vaporeon!)