Copy + paste from a same-name topic on another site that I recently replied to:
I've considered Noko (my Shiny Dunsparce) my signature Pokémon for the past seven years. Inspired by childhood memories of vainly trying to evolve Dunsparce into Gligar, I chained him on Pearl some Christmases ago (without Repels or any real knowledge of the PokéRadar's in-depth mechanics). Fortunately, without trying, I got him with the perfect Nature and Ability combo. Remakes of my favorite games were on the horizon, so I waited until the release of HGSS before raising him where I swapped him with my Starter ASAP. I eventually taught him Headbutt via Move Tutor to maximize his Para-Flinching abilities and considered him my most-cherished Pokémon.
But things change.
I've already considered Wartortle my favorite Pokémon design-wise for a while and finally managed to SR a Shiny one earlier this year (with minimal effort), which I used as my partner in a new file of SoulSilver before importing her to Gen. VII. I realized a couple days ago that I really enjoyed using her, even just seeing her come out of her Poké Ball, more so than I've felt with Noko in a long time. That's when I started rethinking things and decided that she, Torrin, should be my new signature Pokémon and favorite Pokémon overall.
I'm not sure how long this will last, though. I've been really into moths these past couple years and have been trying for a full-odds Shiny Mothim most of this one. After all this time, if I do get one, it wouldn't be too surprising if became my new signature.