Obviously not gonna happen, but meh...
I'd nominate the starter bugs above all. Oo boy, third stage and do they ever suck >_> No chance for redemption there...
Not to mention all those crappity normals who got even crappityer babies... ie Wiggytuff, Clefable, Azumarill, etc.
Hmm... maybe a few others, just for the heck of it (as as sort of gimmick). After all, evolving doesn't necessarily mean your stats increase (Scyther -> Scizor)
For example, porygon Z could evolve again into a different form, with the same stat total but a different allocation (perhaps a physical form).
I dunno... that's all the ideas I have. 4th evos really aren't that necessary.