I always pick my starter according to the design of the evolution line I like most, not for its viability in battle :v
So wellp.
Kanto- Charmander all the way. I really really like the char linem always have from the start so that's not even a contest there
Johto- Cyndaquil. I like all 3 starters, except chikorita's line is meh for me, but Toto line is cool too. However, I've always liked the Cynda evo line the most, especially Quilava. And in pokemon crystal, the sprites for the whole line looked badass
Hoenn- Well shiet. I'm really torn here. I mean the first one I chose was Mudkip because Mudkipz. But I also really like Treecko's evo line so I often just balance my playthroughs with picking either one. For ORAS I went with Treecko because hulkpert is ehhh.. no
Sinnoh- this has got to be the generation in which I literally can't say I have an affinity to any of the starters. In all my playthroughs I pick Chimchar because that's the one whose evolution line I don't hate, but I don't really like it either. I liked Piplup but not its evolutions ..and then the anime made me hate piplup too
Unova- Hoenn all over again. Torn between Snivy and Oshawott. I love Serperior and Samurott both so I try to balance the choice for these as well. My first choice was Snivy though.
Kalos- Wellp. When I saw the 3 starters, I was on team Fennekin immediately, cuz froakie looked ...nothing special. Then we saw Braixen and Frogadier. That solidified my choice. I really liked the foxes so far..and then they introduced the final evos i: I can't say I like Delphox sadly, but Greninja was really cool and wellp. My choice went to Froakie in the end. (Chespin lost me as soon as I saw the rest of its evos)