Fuck yea, I love answering this:
Gen 1: Venusaur: Viable w/ or w/o it's mega and it has a cool design. Bonus Points for being the starter no one really cared for
Gen 2: Typhlosion: Was the starter of my choice in HG and using it again in Reborn made me re-kindle my love for it. Feraligatr is a close second though
Gen 3: w/o a doubt Swampert. Despite Blaziken being stupid op and using Sceptile in the Gen Threemakes. Reborn made me love Swampert due to how useful it is and how many times it has come in clutch, living hits or dishing them out. I fucking love this Pokemon, my 2nd favourite Pokemon overall
Gen 4: Another hard one, since they're all good. But Empoleon wins due to nostalgia reasons. Being the stupid kid I was and refusing to train up anything else but my starter, I struggled against Gardenia. So I levelled up my Prinplup all the way up to an Empoleon via level 16's and beat the gym. Those were good times, when I was stupid overlevelled for the next few parts of the game
Gen 5: Samurott: All the starters in this gen were super "meh", but Samurott takes the cake due to, once again, nostalgia reasons. Played White 2 before White/Black, picked Oshawott which would then become a Samurott and my first level 100 Pokemon
Gen 6: Greninja. I really like the other two, but Greninja is just too cool and good to pass up.
Gen 7: At first, it was Decidueye due to purely design. But despite using it for S&M, US&UM and Rejuvenation, Primarina takes it. This is purely because of Rejuvenation, where Popplio was randomly given to me and from there, I saw how good the Popplio line was. Primarina is amazing, high Sp. Atk and Sp. Def and has a pretty design.