Kanto - They're all loveable but I gotta say Bulbasaur
Johto - Cyndaquil
Hoenn - Treecko (Sceptile is my favorite final evolution of any starter)
Sinnoh - Piplup
Unova - I have the least amount of opinions on these, but Snivy i guess
Kalos - Fennekin (but not the rest of the line)
Alola - Litten...