My Gyarados normally. Rarely it is the starter, as I normally see those with a more calm personality, and not as leaders (same with my Crobat too). In the aftergame, it's a frenzy between my D-Nite, Gyarados and Gengar, which are basicly "competing" for the lead.
A while ago I even went to the E4 with the three of them and supporters (my female T-Tar, a recently trained Blissey and my Lapras) and counted to see which one got more kills in (all level 80). This was in FireRed, so obviously, Gyarados swept Bruno, Gengar pwned Agatha and D-Nite dealt with Lance (with some aid from the Gyarados, for the ice and stuff). They all smacked Lorelei across the face, since the three of them have Tbolt, and the Dragonite has fire, so he dealt with Mamoswine and the Jynx. Same **** with Gary, but to a lesser extent. Ended up being a tie between my Gengar and Gyarados, with the Dragonite being two kills short.
All of them are boss though