Gen 1: Grass, because Bulbasaur makes the game easy-mode. I've tried all three starters, and Bulbasaur is just the easiest one to work with.
Gen 2: Water. It was initially going to be Grass, but then I discovered that Meganium is weak. After many resets and playthrus of Gold and Crystal, I finally settled on Totodile.
Gen 3: Fire. Torchic has always been adorable, and Blaziken has always been awesome. His HA and his Mega make him incredibly powerful!
Gen 4: Fire, because they kinda forced it upon you in DP. Platinum kinda fixed it, but they still only gave you a handful of additional options. It was OK, tho, because Infernape was actually better than Blaziken was.
Gen 5: Grass AND Water. I initially got W1 and played as the female trainer, who started with Snivy. In W2, I played as myself and picked Oshawott.
Gen 6: Fire, because foxes are awesome, and mages are incredibly powerful classes in most fantasy RPGs. It's a shame Delphox got the short end of the stick, competitively.
Gen 7: Grass, because (1.) I haven't picked the Grass starter (for my main character, anyway) since Gen 1, and (2.) Rowlet is adorable, and Decidueye is amazing! I was not expecting Grass/Ghost!