Treeko's line is the worst because they are ugly and there are better grass types, IMO.
Anyhow, here's my opinion on the starters:
Bulbasaur- Adorable. Inysaur- Kind of cool. Vensaur- How the hell did cute little Bulbasaur turn into an obese toad-cat?
Squirtle- Cute. Wartortle- Almost as cute as Squirtle. Blastoise- Why is there a giant obese blue woman with canons coming out of her back sitting on my evolving Wartortle?... Oh.
Charmander- Cute-ish, I've seen cuter. Charmeleon- Meh. Charizard- ...Meh.
Chikorita- OK. Bayleef- OK. Meganium- Would be darling if she had a more feline figure than dino figure.
Totodile- I almost died it's so cute. Croconaw- Looks like a little fat caveman. Feraligatr- Cool until you look at the jaw.
Cyndaquil- Cute, but needs open eyes. Quilava- Cool. Typhlosion- Kind of cool.
Treeko- Ugly. Grovyle- Pretty color scheme, ugly design. Sceptile- Ugly.
Mudkip- Excuse me sir, you have rubber gloves stuck to your cheeks. Marshtomp- Oh hey, there's no gloves on your cheeks anymore. Now they look like.. Nevermind. Swampert- Ugly.
Torchic- Hideous little bugger, aren't ya? I feel so sorry for you I want to euthanize you. Combusken- Looks like a menally challenged chicken. Blaziken- Bell-bottoms, talaons, and a beak that goes up his face? Ugh.
Turtwig- I know this line is going to be trouble. Grotle- Oh God.. Torterra- Not as bad as I expected! Still hideous.
Piplup- Pretty cute. Prinplup- Ugly. Empoleon- Gross. And there is so much potential for appealing design with a penguin -sigh.-
Chimchar- Cute, I'd probably love it if I were a monkey fan. Monferno- Decent. Monferno- Decent. I might actually like it if it weren't for all the 'stuff' (The gold swirls, the shirt-thingy, the giant red eyebrow.)
Snivy- Cute. Servine- You would look better with no limbs. Serperior- Awesome.
Oshawott- Would be adorable without the big nose, but at the same time it adds to uniqueness. Dewott- Oh my god, so cool! Samurott- What the.. How, just how could we go from cute otter to cuter otter to this crap? It doesn't even look like it belongs in the evolution line!
Tepig- I smell BBQ pork. Pignite- Did Ms.Piggy get 3rd degree burns somehow? Emboar- Dear god, you're ugly.