First time on Ruby, Mudkip. Two of my friends and I got the games just about the same time, and decided on starting our journies at the same time, where each of us would choose a different starter than the others. I ended up being last, and had to choose Mudkip, although I grew to really like it, and it became my primary Pokemon, although not surprising considering how strong it is. Although, while I do like Mudkip, Treecko comes just a little above it due to awesome its design is.
First time on Leaf Green, Charmander. This was pretty much tradition. Charizard was all the hype back in the days of R/B/Y, and despite not appearing as powerful with all the other, stronger Pokemon from other generations stealing its spotlight nowadays, it was still a favourite of mine when I first got a remake. I did have lots of trouble beating the first two gyms though, as I didn't catch any other Pokemon than Pidgey on my way up to the first gym, so that cried for excessive grinding. Despite Charizard being an old favourite, I still tend to use Venuaur the most, just because it's easier to plow through the first part of the game with it, but my friend and I did breed hundreds of Charizards at the point when we discovered breeding though lol