In PvP mesmer/guardian (sustained fight kings) > warrior/thief/ranger (burst kings) > necromancer (requires too much time to get "started up" but quite powerful once done) > engineer (situational, excels in mines and point defence allowing for very powerful 1v1 and even 1v2 in some places but lackluster without support or point to defend) > elementalist (great support, terrible at everything else).
In PvE, it's a bit of a different story, mesmer falls into the middle of the pack, guardian/warrior are basically kings due to having both damage and best tanking ability in one package, thief is great burst but a bit squishy on frontlines, necromancer is great when alone but tends to overlap with other condition damage players, elementalist has great healing and control but worst survivability of the lot coupled with lackluster damage and ranger becomes a fairly weird ranged damage/healer mix due to lack of places to use first strike.