My strongest Pokemon was Empoleon being at Level 64 in my Pearl playthrough (when I was also very young, like 8 or 9). I was super scared of battling, as I didn't want to lose and didn't know what I was doing, and just kind of used my starter throughout the entire thing. Still couldn't sweep Cynthia with it, since I only had a mono-Water moveset and the second strongest Pokemon I had was a Palkia at Level 56 XD Yea, I restarted the game completely because of how dumb I was.
As far as I remember, the strongest Pokemon I have right now is my Dunsparce in HeartGold. Despite my three starters (Meganium, Feraligatr, and Typhlosion) being pretty darn good themselves, Dunsparce was a powerhouse and swept Whitney's team with just him. XD It was amazing and I was glad I had the patience to catch it (despite the 1% chance in Dark Cave). Hell, my Dunsparce was 5 levels under my entire team and it still did better than some of them.