Gen 1 (Red): now I will be honest, I still haven't finished off my Red file because I got bored of all the plain graphics and tedious walking very fast. However, I do still have a clear favourite Gen 1 starter, and that is Bulbasaur. It's just so cute an adorable!
Gen 2 (HGSS): Cyndaquil was who I chose for HGSS because oh my god it is adorable, but I'm also a huge fan of Chikorita too since I can't resist those eyes.
Gen 3 (ORAS/Emerald): Torchic, Torchic all the way. When I first bought ORAS as my first Pokemon game I was initially stuck on who to choose, but then I remembered that in the video I watched that convinced me to buy it, it was mentioned that Torchic is a little OP. And, since I had just started playing, I thought "hey, that could be helpful since I have no idea what I'm doing!". Best choice ever. I do love Mudkip though because of its derpy face and, of course, the memes.
Gen 4: I'm always torn on this one between Turtwig and Piplup. Piplup is a penguin which is like one of the best animals ever, but Turtwig has that adorable little derpy face similar to Mudkip's. In my copy of Platinum I chose Piplup, but I can never decide otherwise!
Gen 5: jee it's another conflicting choice between the Water and Grass starter! When I first bought White and White 2 I chose Oshawott instantly because it is cute as heck, but in my second playthrough of White I chose Snovy instead, and ooohhh my god I love Snivy. I just can't choose!
Gen 6: now since I played X for the first (and currently only) time as a Wonderlocke, I didn't explicitly choose one of the starters with the intention of using it or growing attached to it. I used to like Greninja but the anime gave it a sour taste. As for the other two, I'd say I'm conflicted. Chespin is adorable and its fully evolved form looks great, but Fennekin is exactly the same. Again, I can't pick!
Gen 7: Litten. Litten all the way and nobody can convince me otherwise. This thing is my spirit animal. A cat, with the Fire type, that's introverted and kinda likes wrestling? Give me. Also helps that it was female first time in SM. I'm definitely choosing it for USUM too!