Kanto: Bulbsaur. Not only could you easily take down the first two gyms with him alone, his bulk, access to status conditions, leech seed and substitute makes him a dangerous threat. Factor in his new Mega with Thick Fat to allow him to deal with two of his weaknesses, and even everyone's precious Charizard will find it difficult to bring down.
Johto: Cyndaquil. Nothing all to special, I just liked him better than the other two. You know, for reasons.
Hoenn: Torchic. I started with Red/Blue, but skipped Johto. When I got back into Pokemon, I took Torchic to start with. Torchic single-handedly took me from a casual interest to a strong interest. I kept Blaziken with me from beginning to end and never once regretted my choice. And when Blaziken got the godsend that was Speed Boost, followed by a mega evolution....well, just know if you ever face me, Blaziken -WILL- be on my team, and she will certainly surprise you.
Sinnoh: Infernape. Much like Blaziken, it was the better appearance of the other choices, and while the novelty of fire/fight starter was surprisingly carried over, I didn't mind it. Add to the fact that he can learn Stealth Rock and Mach Punch, and I had one solid Pokemon on my hands.
Unova: Snivy.Snivy was much sleeker than the other two, the fire/fight combo was really wearing thin at this point, and while I do like Oshawott, I liked Snivy more. Probably the first time I ever actually liked a snake. While not as reliable as Torchic, she certainly held her own.
Kalos: Fennekin. I have always loved foxes and canines, and when I saw a fox starter, my vote was sealed. Chespin's pretty awesome. Froakie, he has a gimmick in Protean, but one good hit and he's gone. Fennekin might have a stricter movepool, but she can take a hit rather well, and the few moves she has grant a decent amount of coverage. And I love all three of her stages, which can't be said of any other starter in my mind.