Your oldest 12 mons
My oldest 12 are from my first month July/August -16.
My oldest is my Snorlax, got him in my first 10 km egg at the end of my first week (did not know how lucky I was). For several months he was the backbone of my gym team together with my Slowbro.
Out of the 12 still hanging around, I have maxed out 7 to lvl 40.
I am a bit sad I deleted my starter Bulbasaur to get candy to evolve my first Venusaur.
Answers
Fun question. I was actually surprised to see some of them where they were. My Machamp is still my main Champ. He has the legacy karate chop/cross. That Rhydon is my highest double ground still, and my Vaporeon has always been a key attacker and defender. That Tangla was my main grass attacker and my Submission Poliwrath and Rapidash, to a lesser extent, were early Snorlax counters. My Slowbro was my long time Dragonite slayer.
Starter Squirtle, Blissey which was the first mon I ever gave Stardust to and my first Wonder IV Pokemon (aside from the Cleffa), my delightful Porygon2 that I've walked for an eternity, my terrible-moveset Jolteon that was the first Pokemon I was able to actually take on enemy gyms with, and my Igglybuff, which is a fascinating story.
7/10/2016 Gyarados with Bite/Twister - 1st in my Community to evolve one.
7/15/2016 Rhydon
7/19/2016 Omastar - Rock Throw Ancient Power
7/24/2016 1st Dragonite - Steel Wing Dragon Pulse
7/24/2016 just a good Jolteon
7/24/2016 Karate Chop Machamp
7/25/2016 Body Slam Snorlax
7/25/2016 Shadow Claw Gengar - maxed
7/28/2016 1st Muk
7/29/2016 1st Electrode
7/29/2016 Powered Up Blastoise from a Squirtle I hatched
7/30/2016 Zen Headbutt Solar Beam Exeggutor