I think the earliest one I can really remember who I had a fondness for not driven solely by force of novelty is a Blaziken I had in Ruby version. I started a new file at around 10:00AM, and played it all through the day, only finishing my play session at around midnight. By that time, I had reached Norman and my Blaziken was already, well, a Blaziken, and blasting effortlessly through the game. By the end of the playthrough, that Blaziken was leveled in the 70s and aided only by Groudon, although I... think I might have gone to grab the Regi trio for some additional backup facing the Elite Four? I can't recall.
The second who comes to mind is a Zorua I got over GTS in Y version. She was clearly a reject from some breeding program, and I felt really bad for her, so I ended up with my first-ever (and still only, so far) seven-Pokémon main team. She's since grown quite strong, and evolved into a Zoroark, and I'm very proud of her! Honestly, I'm contemplating using a save editor purely to give her a name, since she doesn't have one and I obviously can't give her one in-game since she's traded. She certainly deserves it... she's a good girl.
Most recently, Dulcie, my Alcremie in Pokémon Shield! I wanted to try using an Alcremie, since it is a cute little Pokémon, and I love its sort of "magey" theme, but she ended up positively lethal! She's got fantastic special sweeping potential (and can even do physical, too, now that I've given her Psyshock with which to whack opponents' Defense with her mighty Special Attack stat), and can even make a physical tank of herself with Acid Armor! If I had to pick just one Pokémon to say is the singular MVP of my team, it'd probably be Dulcie, but she's actually part of a trio! Hanabi the Cinderace, Dulcie, and Cascade the Vaporeon are pretty much the heart of my team; they've been with me nearly from start to finish (or literally, in Hanabi's case), and have been solid all the way through! They're my A team within my larger team, really, although most recently that "A team" has been joined by Tellus the Tyranitar, too.