In no particular order
1.- Rhyhorn: I'm not sure why but I've always liked it, I think that most probably its because rhinoceros are my favourite animals but I feel theres something else : o and I always hate when the pokedex says that they're like super dumb and stuff, idk I always imagined them like relaxed until you bothered them
2.- Snorlax: How can you not love him? hes a lazy big pokemon who will do anything for food and sleeping, plus I feel he sort of represents me xD
3.- Mewtwo and Mew: Idk but I've always liked their story from the first movie, I mean mewtwo being the poor pokemon who was only made by human kind to be studied, and mew the playful pokemon who just basically wants to help without really hurting anybody
4.- Torterra: I simply love that typing combination *-* grass and ground where some of my favourite types from the beginning
5.- Charizard: I liked the charmander from the anime a lot, and for a while I stopped watching it (idk why, I watched it when I was really little) so when I started watching it again (still little) it seems I sort of jumped right to when he had already evolved to Charizard but wasn't too egocentric so yeah it was funny how he always burned Ash in a like friendly way (never thought I would make such a phrase xD) and he would beat everyone plus in the first movie he was cool too
6.- Tyranitar: He looks like he could be a Kaiju and I tend to like Kaiju movies
7.- Claydol: Emerald was the first pokemon game where I played like thinking about it (Second pokemon game in general, firered was the first and I couldn't even beat the Jynx from the first Elite4 xD) and he appeared in the only gym of all pokemon games who have ever gave me any problems (not counting Pokemon Reborn duh hahaha) so yeah I sort of liked him from the beginning
8.- Chandelure: He was the one who came to fill my firestarter space when I was playing Pokemon Black, idk why but on my first play of all pokemon games I always choose the fire starter and while I was playing Black I thought that Emboars evolutionary line and learnset sucked so yeah Chandelure replaced him and it has been the only time I've done it xD
9.- Tentacruel: Idk hes just cool, plus I always imagined him bigger than what he actually is so yeah even cooler for me
10.- Gengar: Ghost types are some of my favourites and I liked how they pictured them on the series when I was little
So yeah I tried to make a top ten (actually eleven but meh) but I feel like I'm leaving plenty outside (Onix, Barbaracle, Avalugg, Jellicent, Golem, Muk, Magmar, Magnemite, Hippowdon, Bronzong, Rampardos, Zapdos, Tropius, Camerupt, Cubone, Weezing, Dusclops, Lapras and probably more), and through this I came to realize that most of my favourites are on the first generation