My least favorite starter was Chikorita. In Pokemon Adventures, it was OK (but slightly annoying), but it's so boring and iritating!
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Looks-wise I always found Chimchar and it's evolutions to be unremarkable. It is the only Sinnoh starter I don't use. I just felt like Infernape was an okay Pokemon in terms of moves and typing but Blaziken beat it to the post as the original fire/fighting starter and could do everything Infernape could and still have a design I liked.
Probably Tepig and Froakie. Tepig's evolutions are kinda ugly to me, not to mention that they're the 3rd fire/fighting starter evolutions in a row. Froakie itself is also ugly in the games, but his anime artwork is a bit more bearable. I also don't really mind Frogadier, but I'll need to see the final evolution to base my final opinion on the line.
Was going to vote for every Sinnoh starter, but I unmarked Piplup at the end because Empoleon by itself is ok. Chimchar and Turtwig though... are just blah. Same with Oshawott.
I have a love-hate relationship with Snivy (Arbok is seriously more powerful than Serperior...) (It learns terrible moves and doesn't even get Earthquake like the other starters) (Servine is possibly the stupidest middle evolution ever) (But Serperior LOOKS cool) (And it's like a grass type mate for my Milotic) etc etc. So sometimes I hate him, but I didn't vote him as least favorite.
I don't like Treecko and Mudkip. I'm pretty sure I picked one of them (Treecko?) because for a while I had the habit of picking a Grass or Water starter thanks to the Gen I games and it came in handy when battling Roxanne. I don't hate but I'm not a fan of Snivy. If we're talking evolutions I don't like the evolutions of Snivy, Torchic and Tepig, which is a shame because I like the latter two... I guess that not evolving them is always an option.
Pikachu- What the crap, man? Probably the worst Pokémon, let alone starter, in history.
Plus the Gen V starters, because Tepig evolves into ANOTHER Fire-Fighting and has an awful movepool, Oshawott has paint by number stats, and Snivy is all around awful.
(Dis)Honorable mention to Fennekin. Again, what the crap? It's Vulpix with its awesomeness removed!
Chikorita, the only starter I have never attempted to use in any capacity
I'm afraid I don't like most of the starters beyond Generation II. I hate to sound like somebody who only loves Pokemon from the older generations because that is soooo not true (for example, Piplup is my favorite stage one starter, and Samurott is my favorite final stage starter altogether), it's just that it's kind of hard to decide. I think I'll have to go with Tepig. Tepig itself is actually pretty cute, but its evolutions are... Let me just say I can't find any appeal in them, sadly.
All of the Sinnoh starters. Honestly? Not a great generation for starters. I didn't really find any of their designs interesting, particularly in their first forms. (The following opinion is based solely on designs rather than stats, by the way) Chimchar made me want to punch it in the face when I first saw it, and Piplup made me want to from its annoying character in the anime. Turtwig was... okay. But it just had such a big head proportionate to its body that I was annoyed by that. And I'm not all too fond of Grotle. (Though Torterra is... okay, I suppose.) Chimchar's evolutions are decent, particularly getting better at Infernape (and not really being all that great at Monferno), but I still am kinda meh at them. And Piplup? Its middle evo was okay, and I found Empoleon uninteresting aside from its type combination.
It's hard to say that there wasn't really any Pokemon per se that I was overly disappointed with. Samurott maybe? I feel that Oshawott had the potential to become something so much better, and it ended up as something more or less disappointing, aesthetically. Emboar was also...more or less "meh" to me. It has grown on me somewhat, and I won't say that it's terrible or anything, but it's underwhelming.
Gonna go with Chikorita and Turtwig. I always forget they exist. Never was a fan of their designs.
Though, before Gen II came out when I was a kid, I used to draw a lot of Fakemon and came up with one that looked almost exactly like Chikorita, which I was shocked at. I guess that's why I have a hard time believing it's real, since I made a Fakemon close to its design.
Don't like Chimchar and Piplup either, but I like their final evolutions.