1.Oshawatt: I am totally biased because he was my first pokemon. Funny enough, it was this close to being Snivy but I decided to choose cuteness over coolness, even when I would visit Unova again in White 2, I still chose Oshawatt because I just couldn't say no to his freckles and adorable smile. Though I would grow to love other pokemon even more, Oshawatt and its evolutions will always hold a very special place in my heart (excuse the corny phrase). Even in the anime, Oshawatt seemed to have the most personality out of Ash's new starters. I can't really give any expertise on any pokemon's battle usage though. ^^'
2.Chikorita: Again, cuteness wins this one for me but also one other personal reason. Meganium is very clearly based off of the brontosaures, which is my favorite dinosaur, so having a really cute flower version of the vegetarian was especially appealing to me. I still have a close attachment to it as its sort of the under-dog of starters and I still hope Meganium gets a mega evolution.
3.Squirtle: This is the favorite pokemon of my uncle (who got me into pokemon in the first place) and my third favorite. Simple, yes, but also extremely iconic and packs a lot of nostalgia. In addition to being another cute, if not mischievous, water-type Squirtle just reminds of how the pokemon community itself has grown over the last twenty years, if that makes since. Like, though first was actually my fourth pokemon game, I was still able to connect with people who had it as their first game. I don't really know how to express it, but it's just neat how Squirtle and the other two first gen starters are still relevant in the modern games.
4.Piplup: I absolutely love penguins and I hope there will be more penguin pokemon in the future but for now we have this cutie. Piplup was my favorite pokemon back before I played the games and I just watched the Sinnoh anime, so its close to me for that reason, too. My only gripe is that I don't really care for Empoleon, actually...
5.Popplio: Adorable pokemon turned beautiful? Yes, please. I like Popplio for similar reasons why I like Oshawatt, but there's also the additional bonus of having a really graceful evolution (as apposed to the stoic Samurott) and a neat shiny, in my opinion. Popplio is probably also based off the Irish mythical creature, the Selkie, which is pretty cool.
(I know these opinions are SUPER subjective but I hope it will be interesting to read, nevertheless. I, ironically, had more solid objective opinions on the gens but I missed my chance to post reply so...oh well.)