Hitmontop for me. I used one for the first time in an old playthrough of HeartGold or SoulSilver, don't remember exactly which. It was from the Tyrogue you get from the Karate King in Mt. Mortar. That was the only way to get one of the Hitmons in that game, and then if you wanted to get Hitmontop specifically you had to watch the growth of your Tyrogue to make sure it had perfectly balanced attack and defense.
The entire thing felt like a really cool sidequest that took advantage of the preexisting mechanics of the game, where you had to actually pay attention to the way your Pokemon was growing and train it accordingly, which made it pretty memorable. It made the Pokemon feel unique and special, and the work you had to put in if you wanted to get a specific evolution, especially Hitmontop, made it so you got attached to the guy in the end. It's pretty cool to see something like that even as early as Gen II.
I used that Hitmontop all the way through Kanto and even though I'm pretty sure it never was the strongest in that team, it was almost certainly the most reliable. I still remember the whole experience, it's one of the things I remember the most from that playthrough, so Hitmontop holds a special place in my heart because of this. I've also grown to like it the most in terms of design, it's got that Gen II charm. And I like its playstyle the most out of the three as well. Just an all rounder, balanced, humble support focused Fighting type. Nothing too flashy, but like I said, reliable.