My first attempt was my Lucario, Shenzi, in Pokémon White. I thought I got her to lv 100, but after checking while sending her to Pokemon Sun, it turns out I only ever got her to lv95. Pretty close though.
My first successful attempt were my Female Pyroar (Fliker), and my Yveltal (Shard) in Pokémon Y.
Since then I haven't gotten any of my Pokémon to lv100 yet, though I've had quite a few I've gotten get close (like my Scizor, XY event Blaziken, etc). I'll make that one of my goals in Ultra Moon.