Now that there are 27(+) options, it's hard to decide on just one.
Quite satisfyingly, my favourite starter-type alternated across generations, in order;
Bulbasaur, Cyndaquil, Mudkip, Turtwig, Tepig, Froakie, Rowlet, Scorbunny…
But then Fuecoco came along and changed that pattern. It broke the trend of being overly-humanised, gets a cool signature move in its final for, and is generally just great.
So I guess they'd be my favourite starters of each generation. It's honestly too hard to decide on a favourite overall.