Although there were a number of Pokemon such as Starmie to which I have a strong loyalty, I'd say Volcarona was the most fun to use and my favorite Pokemon in Gen V OU. It could fit on so many different kinds of teams: balance, HO, sun, even rain. But the reason why it was so much fun was because a lot of games centered on whether or not you could give it the one chance to get it in safely, set up, and wreak havoc. It had a few weaknesses (STEALTH ROCK being the biggest and most obvious), but these risks taken and the plays made to avoid them (when you could bring something in to Spin, outplaying or being outplayed by spinblockers, bringing in weather, setting up Subs) were what made the game fun every time I used one. It was powerful without being overpowered.
Volcarona @ Focus Sash / Life Orb
Ability: Flame Body
EVs: 4 HP / 252 SpAtk / 252 SpDef
Nature: Modest / Timid
- Quiver Dance
- Fiery Dance / Fire Blast
- Bug Buzz
- Giga Drain / Hidden Power Ground
My HO set, and sometimes my HO lead. Although early set-up is usually frowned upon, being able to come in without worrying about Rocks and crushing souls from turn 1 was a lot of fun. I started running HP Ground over Giga Drain when I realized that Heatran was just such an anti-meta Pokemon and so common that I needed to run it just so my sweep wouldn't end prematurely; also, for some reason, no one expects HP Ground on Volcarona.
Volcarona @ Leftovers
Ability: Flame Body
EVs: 252 HP / 252 Def / 4 SpDef
Nature: Bold
- Quiver Dance
- Fiery Dance / Fire Blast
- Bug Buzz
- Roost
My favorite set, because I really favor stall and balance over offense and HO. The given EVs hit 374 / 251 / 246 defenses, and the ability to do something akin to Calm Minding while Speed Boosting and Roosting off damage made for a dangerous set-up sweeper. You needed to make sure its counters were out of the way first, but again, that's what made so many of the games in which it took place so much fun. It required thought to get to a desirable end result. That pretty much summarizes Volcarona: a strong Pokemon which still requires thought and prediction to use.