Also missing Honchkrow and Mismagius in the poll. Though neither of them are my least favorite.
Toss up between Probopass. Magmortar, Lickilicky and Ambipom for actual worst strictly in terms of design.
I actually like Rhyperior's design, it just feels like an unnatural step up from Rhyhorn and Rhydon - perhaps they could incorporate that into the games' lore somehow. Similar feelings but to a lesser extent for Magnezone, Togekiss, and the aforementioned Mismagius.
Weavile's "headdress" is stupid looking but otherwise the design works.
To end on a positive note, Roserade, Mamoswine, Tangrowth, Honchkrow and Yanmega are the opposite; I like their designs and they make sense in the evolutionary line.
Most of the final evo's grew on me. Especially Magmortar since I used one for my Black 2 team and it was surprisingly good. Probopass, Electivire and Ambipom grew on me because I used them in my Pokemon Platinum nuzlocke. And Lickilicky... I don't know. It's so silly looking, I can't be upset with its design!
But there's one evo I really don't like: Togekiss. I love Togetic a lot. But Togekiss' design doesn't do it for me. The weird, oval shape just looks so off to me. I would've preferred more of a bird look than... a bath toy? I don't know how to describe it.
As for great final evo's. Roserade and Weavile take the cake! Roselia and Sneasel needed good evolutions and they certainly got them!
Wow, I just realized how much I dislike the final evolved forms of old Pokemon that were introduced in Gen 4. I guess Probopass is the ugliest of the lot, but Tangrowth, Yanmega, and Magmortar are also up there on the ugly list.
Magmortar is a pet peeve of mine since Magmar has one of my favorite designs (an unpopular opinion, I know) and I feel like they totally wrecked its evo. I hate that it no longer has a long tail and that its huge hanging gut is spreading its skinny legs so far apart. It went from being a kool Chinese-dragon-rock-singer to just a fat brute.