For pvp (as mentioned earlier)
Lv60
(lets pretend these are good at their class people) this is 1vs1 in a fair fight, not random gankings
healers = mystic/priest
Berserker -> healer classes, archer, maybe sorc, maybe warrior
Slayer -> berserker, slayer, lancer, archer
Lancer -> berserker , warrior , sorc, possibly healers, possibly archer
archer -> healers, sorc
sorc -> maybe berserker , slayer
warrior -> slayer, healers, archer , sorc , maybe berserker
healers -> maybe any
most all of which, I've seen through my travels
Note the strongest combo is a slayer+healer. if both are good, they can usually take on 5 people and win.
Anyway in PVE terms... lv 60 of course! Each class has its advantages and disadvantages.
overall, for the "best defensive" and best offense is berserker.
Why? axe block. it can be used constantly, it has no cooldown, and it can be kept up when "alot of shit" is happening. The thunder strike charge time, gives you room to run out while charging avoiding potential "turn and cleave" attacks. The evasive roll gives you extra room to chase down/evade an attack when running up to not waste that thunder strike.
It is probably the most forgiving in "self defense" and potentially the highest in melee damage ( attack power goes to near 40,000 with a +power gem, enrage mode and the attack buff and a lv60 epic axe). on a enraged mob with 3 +crit gems from the back this gives potential to crit toward 240,000-260,000 when the stars align and a near once in a lifetime miracle happens, and with axe block, if the boss decides to come rape your face.. just hold that block down...!

(saved me so many times on lance boss, he spams his lance crazy fast, hits about 4-7K through a 20,000 damage absorb axe block)