You know, in thinking about it I've rarely used my favourite first which is really weird when you think about it.
In Gen 1 first starter was Squirtle. People I knew at the time recommend using it so I did. I think I got to the 3rd gym before being like "I really like Charizard. I'm going to start a new game and with Charmander, but not save". At the time though I didn't know the PC auto saved and I used it so I did end up getting to use Charizard.
Gen 2 I was on the Fire train so auto picked it. I might prefer Typhlosion's design to Feraligatr's, but I REALLY like Feraligatr as a Pokemon (thanks Sheer Force). I think I'd also rather use Gatr even without its HA.
I really like all the Gen 4 starters and to mix things up used Turtwig. I'd only used a grass starter once, Bulbasaur in LG, but that is the only time I never played a game through (I just beat FR).
For Gen 5 I've never even used Snivy between my two play throughs. I used Oshawott to use a water starter (I'd only ever played through with Mudkip), and always use the gen's fire starter. I think I might have liked Samurott best at the time but I've both gotten higher on Serperior's design and it's HA is also insane.
Gen 6 I really loved Fennekin (I prefer it to Froakie) and went with my first impression since I still really like Delphox.
Thinking about it, I didn't really use any of the Gen 1 starters till FRLG (considering I played Yellow, and didn't use the gifted starters), even then I went with Bulbasaur first, though it was Squirtle that reached level 100 (Hell, I even went with Squirtle in XY).
Gen 2 (and the remakes), I was all about Totodile, and its HA is not too shabby either.
Gen 3, even though I picked Mudkip (type-wise for final form), I do like the designs of Blaziken and Sceptile
Gen 4, I will admit I used Infernape last even though I do like its design. Like Mudkip, it was Torterra's typing that drew me to Turtwig.
As for Gen 5, I haven't really used Snivy at all even though it looked cool as-is
Gen 6, I liked Chespin's design though I could not deny how cool Greninja looked, leading me to use it in a subsequent run.
And Gen 7, Litten grew on me very quickly