on official / low rate, blacksmiths with lots of money are also good mvpers, they dont improve too much in rebirth cos max overthrust doesnt stack with mammonite. alot of their success is the damage they take as well as recieve, which helps mvp. be prepared to burn white pots like no tomorrow. id have to agree that snipers are good because they can mix element and cards easily, and a good wizard is always strong PVM anyways. champs go without saying really (hate champ XD)
pvp wise there is no best, all have str and weakness. champ is easiest to master in the most part, due to 1 hit kill asura and hugely powerful skills. the best all round pvper for many situations i have found to be vit spear knight/lord knight as u dont really have to many weaknesses, and enough HP to really have time to out tactic ur opponent. personally i always pvp with blacksmith and WOE with my sacrifice/GC/reflect/heal pally on IRO. (very effective build for teamwork - sacrifice + heal is awesome)