Gen 1 (Red) - Alrighty then. Now, Red wasn't my first Pokémon game (it was actually Pearl) but I did play it as soon as I could get my hands on a Game Boy. Understandably, I chose Charmander, my favourite of the three and the box Pokémon.
Gen 2 (Silver) - I chose Cyndaquil in Silver. I had already played SoulSilver, and I decided to change it up a bit, and went with the Fire-Starter. I also thought that Typhlosion had the coolest design of the three final-evolutions.
Gen 3 (Sapphire) - Treecko was who I picked! I was really fond of Grovyle (Thank you Mystery Dungeon!) and I loved the speed that the whole line held. While I was a fan of Mudkip and Torchic, neither Blaziken nor Swampert ever appealed to me much.
Gen 4 (Pearl/Platinum) - Ah, memories. I remember choosing Turtwig after a long period of deliberation in both games. He looked the cutest, and I had no idea what was in store. Heck, I didn't even know that Pokémon evolved at this point. But I never really liked Chimchar, who's design just repelled me, and while I did like Empoleon (Water/Steel woo!) I never picked Piplup.
(SoulSilver) - When I played this, I wasn't sure who to pick. Cyndaquil was cute, but I didn't like the eyes (to start with, now I absolutely adore Cyndaquil), and Totodile, while a cool concept, seemed a bit too obvious. So I chose Chikorita, keeping with my preference from Pearl. It was cute, and quite a challenging typing to use, being weak to Gyms 1 and 2. And I was lucky enough to get a her. On my first go. I didn't know it was a low chance; I thought it might have been made to only be female.
Gen 5 (White/White2) - I got White as a birthday present, and I had no knowledge of any of the new Pokémon. I had no idea what to expect, and I was a bit worried when it came to choosing my starter. The game was fun, I enjoyed the intro, fell in love with Cheren, and opened the magical box. Oshawott seemed cute, kind of, but I always look at the others first. So I looked at Tepig, and knew I didn't want to choose it, no matter how much it smiled at me. So then I looked at Smugleaf himself. Snivy was the one I chose. I knew that that was my favourite, and still is my favourite starter to this day. In White 2 I followed the trend and chose Snivy again, but if I ever start White again, I think I'd go with Oshawott.
Gen 6 - Okay, so I have never actually played a Gen 6 game... Although, I always knew which starter I'd pick. I never really liked Chespin, and it just didn't seem right for a Grass-Type. Froakie seemed kind of cool, but when I discovered that Greninja was basically a diety in the Anime and super-powerful, I grew to dislike it. So I chose the undeniably cutest Gen 6 starter, Fennekin!
Gen 7 (Moon/Ultra Moon soon!) Well well well, my first 3DS game. I had seen the evolutionary lines and knew what to expect. Popplio seemed alright to start with, but I disliked Primarina and the Fairy Typing, which I am now (sort of) neutral about. Litten was cool, but I hate the wrestler theme that it adopted. That left Rowlet, who gained a Ghost Typing, and basically became Robin Hood. No brainer for me! And in Ultra Moon, I'll pick Rowlet again!
So, I've never actually chose the Water Starter, but I have an obvious preference for the Grass Starters!