For me, it was:
Ruby & Sapphire: Torchic & Treecko (respectively)
Emerald: Torchic
FireRed: Charmander
LeafGreen: Squirtle
For RSE, I chose what I did, because I wasn't, and still am not, keen on Mudkip; despite the advantages it holds. Torchic was the first Fire type I chose from first playing a Generation-based game, (since Generation I & II had me choosing Squirtle and Totodile, respectively) Treecko, again, was chosen over Mudkip because of my disliking for it, and currently, I still have never used Mudkip, except for Marshtomp; its evolution, in a Battle Tent.
I obtained LeafGreen, first, and thus, continued my choice of starting with the Water type Starter, since other than Generation III, the Water type was a high-stander for me, above the other types for that Generation, but due to the different Legendary dogs you could obtain, in FR/LG, when I obtained FireRed, I chose Charmander, to obtain the next Legendary dog. (as I had obtained Raikou, from LG, and already restarted with Bulbasaur)