It's funny how attached you get to individual Pokémon, isn't it? I've had several Pokémon that I've loved, but these are the ones who I remember the most.
First up is Quilly the Typhlosion, from Gold. It's been a long time since I played Gold, but I still have my original save file, simply because of this fellow. He was the first Pokémon I ever grew attached to, and he was as tough as nails — I remember that he somehow never fainted in battle, even against Red. The little eight-year-old me got so attached to him that, when I played HeartGold, nine years later, I chose Cyndaquil again and named him Quilly.
Then there's 'Swamps' the Swampert, from Ruby. He was the first Pokémon I ever raised to level 100, mostly through clearing the League over and over again. On one occasion — Swampert was level 79 at the time, I believe — he defeated the Elite Four and Steven singlehandedly, with no items used and, through some insane luck, not a single point of damage taken. When I played Omega Ruby, I chose Mudkip as my starter; this Swampert's name is Titan, and my original Swampert is sitting on White. I need to transfer him up soon...
Third, 'Emmett' the Emboar, from White. Last starter on this list, I promise! My Emboar gets his place here because of his amazing performance against Ghetsis. That Hydreigon of his took out four members of my team — Simisage, Sigilyph, Zekrom, and Samurott — in quick succession, and my Stoutland had too low HP to stand a chance. My Emboar — he didn't have an official nickname, but I found myself calling him Emmett — withstood a Surf with about half his HP remaining, then knocked out the dragon with a single Brick Break before soundly beating Bisharp and Eelektross.
Finally, we have Hanna the Pinsir, on X. I received Hanna in a trade here on PC — a trade that happened when I was browsing the Quick Trade thread and offered to trade a HA Aerodactyl to someone who was looking for one. It turned out that the other user had already managed to get hold of one, but offered to trade me Pinsir anyway, and even offered to nickname her for me. I called her Hanna, and she's very special to me, because she reminds me that there is still kindness and generosity in the world.