Class overview

Class roles
Solo PvE: Blade Wardens are probably the best all around, good armour, damage and cc abilities if you get swamped. Raksha Warriors are very good too, with even better armour and some AoE. If you enjoy PVE as a glass cannon the Fire Mage is probably best (the Dark Revenant is too mana intensive). The Master Summoner does quite well too, but it's more of an all-rounder.

Party utility: Surprisingly enough, the answer is every class! Everyone gets a buff to share, and the PvE (outside istances) is not difficult enough to look for a tank and a healer.

Flexibility: Master Summoner hands down. He can buff, heal, ress, do damage by himself and with his summons (you can have more than one out). Blade Wardens are very good all-rounders too, but they cannot heal or ress.

Solo PvP/PK: Again Blade Wardens probably take the cake here, with good defense, offense, CC and a bubble, closely followed by Stormlords (same stuff, but more group PvP oriented). If you can manage PvP with summons, not only can the Master Summoner break roots, but it's got three minions out at the same time, and he can heal himself and slow the opponent down. The Raksha Warrior has some really good CC also, plus the heavy armour.

Anti-gank: Vajira Guardians and Divine Enchanters are really slow to take down, your opponent is probably gonna get bored way before he can kill you. Master Summoners are quite good too, you can set your summons on the opponent, slow it and run away happily (Summoners break roots).

Mass PvP: Every class is needed here, but Stormlords and Raksha Warriors got a lot of AoE CC. Blade Wardens and Master Summoners are quite good too.

Class utility
(this is sligtly pvp oriented)
 * (* the Master Summoner needs to sacrifice a minion to either AoE stun or dispel debuffs)
 * (** one of the Dark Revenants buffs can only be cast on a single target, but stores their soul so they can ressurect on the spot when killed

not in the table the following special skills:
 * Raksha Warrior can draw their opponents towards them
 * Blade Wardens have an interrupt
 * Dark Revenants can fear single target and AoE
 * Master Summoner Is The Best Class For PvE

If you're interested in a healing class, see also the mana cost and efficiency of heals.