My first Lvl. 100 was Lapras; transfered from FireRed and hit Lvl. 100 while I was challenging friends' secret bases in Sapphire. My Charizard and Tyranitar, also from FireRed, weren't far behind, along with my Kyogre, Sceptile, and Rayquaza.
The highest level Pokemon I ever had in the first games I played, Silver and Gold, was my Ho-oh who I think I never got past Lvl. 94-98 before I moved on to gen 3.
My first Pokemon ever was a Cyndaquil who I think capped out as a Lvl. 70-some Typhlosion before I reset my game and started over with Totodile. God, did I love Gold and Silver. I played through those things so many times... I was so familiar with them that when I imported HeartGold from Japan I had no problems navigating through the game despite not knowing a lick of Japanese outside of ”おかあさん” (okasan). So at least I knew whenever my mom was calling me over the Pokegear!