I wouldn't stick to 1 type, as that's to limiting.
1. Emboar - Technically my first Pokemon (1st one was actually a Mudkip, but it was in a bootleg copy of Pokemon Sapphire, and that copy is long gone and broken, so I don't count it.) one of my favorites, and one of the only Pokemon I've managed to get to Level 100 in games other than XY-on.
2. Primarina - Just became my favorite Pokemon after Sun, was the undisputed MVP of my team. Very well designed Pokemon, and I loved the team so much I raised them all to level 100, just because of how much I played that game.
3. Hippowdon - First shiny Pokemon I ever found in Black 2, pure luck, wasn't even hunting. Shiny Pokemon hunting has become one of my favorite hobbies in the Pokemon games. Didn't get to use him on my champion team, but he became a permanent team member after finding him, (sorry Omastar.)
4. Bulbasaur - First pokemon I've ever seen, was my favorite when I was a kid and getting into the franchise (right around when FireRed and LeafGreen released,) I never used him until Pokemon X, as my first game was Pokemon Black 2, but whenever I play gen 1, or get the option to choose one of the gen 1 starters, I'll always go with Bulbasaur. (Not the biggest fan of Venasaur, Bulbasaur is always my favorite, well second favorite Pokemon.)
5. Groudon - First TCG card I've ever had (Groudon-ex from EX: Crystal Guardians, got it as a gift for Easter around 2006-7.) I still have, and it has served me good. Reminds me of my love in TCG. I bring it to every major tournament I go to.
6. Rayquaza - First competitive TCG deck, old one from BW: Dragons Exalted, (I know this list is a lot of firsts, but what else would I choose.) Didn't do to great, but was still fun and taught me a lot.