Generation I: I suppose is very clear. Charizard, Charmander was the first pokemon I chose when I started playing pokemon. Curiosly, Charizard is the only pokemon know so far to confront Legendary Pokemon on its own? (Groudon, Entei, Kyogre, Articuno, Mewtwo), and participant from many main battles in Anime Series?
Generation II: Welp, if I were to choose, it'll be Feraligatr, has a nice bulky, good Atk, ability, and moveset. Also, it has a Dragon-like appearence
Generation III: Definetly, Sceptile, a fast pokemon, balanced Sp.Atk and Atk. A cool pokemon imo. Also responsible from defeated Tobias's Darkrai in Sinnoh League.
Generation IV: Infernape, yes, this monkey from hell. When played Diamond, I didn't know what starter choose, since tbh, wasn't aware of them, they were new for me. But I choosed Chimchar because I like fire types. A good choice, Was a key when battling E4 and Champion, (Practicaly solo'ed Cynthia). Also, remember that battle of Infernape, vs Electivire? So epic.
Generation V: I don't like any of these tbh.. But if I were to choose, it would be Emboar. (Fav Gen V pokemo are Krookodile and Hydreigon)
Generation VI: Greninja, came to like as soon as came out. Great ability, Sp.Atk, Speed, many movesets etc. I always Charizard over this one, but, when I started playing Reborn, I said "Let's give it a chance", and oh my, what a chance, it didn't dissapoint me. Clear enough to say that Greninja and Charizard are my favorite pokemon over.
Generation VII: Don't know very much about this ones, but I'm sure Decidueye got the place for this. (I didn't like Incineroar, it's like seeing a Dark Type Emboar)