I base my opinions solely on appearance. I don't care about the "it needed an evolution so it would be competitive in battle" arguments.
Evolutions that should have been different - Crobat (how does it evolve from Golbat? Make it a split evo from Zubat), Bellossom (should have evolved from Oddish), all non-original Eeveelutions (where's the ruff? Eeveelutions should have ruffs! Plus Leafeon looks unappealing), Octillery (fish to octopus? No, split the two into separate families), Sceptile (minor design change needed), Nosepass (I like the concept, but needs a design change), Huntail/Gorebyss (how do they come from Clamperl? Split them off).
Evolutions that never should have existed - Lickilicky (give 'tung a prevo instead and call it a day), Rhyperior (Rhydon is impressive enough), Dragonite (ruins Dragonair), Probopass (because c'mon), Rotom's alt forms (because they mean Rotom can't get an evolution now).
Anything I haven't mentioned, I either like (Magnezone, Electivire, Yanmega, Tangrowth, etc.) or at least tolerate (Dugtrio (only because Diglett has that design), Unown).