I'm not one of those who after a certain point can just box away their starters (although I did ditch a Meganium in HeartGold after the Pokémon League in order to bring in my Extremespeed Dratini), yet love both Bulbasaur and Charizard so much I've never ever used Squirtle, despite owning LeafGreen for 12 years now.
I've owned quite a few Togekiss due to getting given the egg in three of the games I own (LG, Pt, HG). Big on dragons too, very often found with a Salamence in Emerald, or a Dragonite and/or Gyarados in LG / HG (not a big Garchomp guy though). Oh, and there's such a nostalgia factor when I re-play Emerald that combines with the fact that they're all good Pokémon that I can easily traverse Hoenn with only Pokémon captured in the first 3 routes in that region (Lotad and Ralts' line in particular are favourites).
Despite that I do try to use different Pokémon as much as possible, aided by the fact I have a storage save in Gen IV so have Pokémon going back to 2011. For example despite a preference towards only using Pokémon introduced with the region I'm playing, I'm finally using Gengar, Machamp and Vaporeon on my current HeartGold team.