guardians are slo assed at 40+ to level solo. painfully so. Not really a call for 2 guardians in a group 1 fills a niche, 2nd adds nothing as all classes out dps them they have no CC or healing ability.
Hunters are the quickest to level, there are lots of them, but no harder to get a group for 1 than any class except Tank/ Healing classes. many instances can benefit from multiple hunters they have decent CC (roots and fear) and their ranged DPS is good.
Captains, are the buffbots, the have decent dps, and can tank reasonably well. (i tank endgame with my capt at times) they hit slow and hard, huge crits (but not often, sure my melee crit is 6%) a captain in a team can turn a wipe into a survivable situation. They use heralds/banners that are beneficial, have various marks to put on target (%age of damage returned as heals, + damage, +threat). Captains main weakness is they cant heal themselves only others (unless traited)
LM's are quick to level their dps is nice, survivability for a squishy class is fantastic due to the fact they are masters of CC. The are the power batteries of the team, filling up fellows and draining the bad guys. And they get pets that tank/ do sneaky dps/ dps on multiple targets etc.
Burglars can Solo Elites and some that ive seen manage Elite Masters. Their Dps in Melee is 2nd only to Champs. when the turd hits though they go down fast. but then most burglars know where their HIPS are (Hide In Plain Sight)
Champs are dps monsters, downside being that in Fervour stance they cant dodge/evade/block and take -30% heals (excepting their class heals) levelling is fast, "good" champs always in demand due to the fact they can interrupt mobs without breaking a sweat, and can tank groups better than most guardians ive seen (but not as well as a good guardian) Traited can us eHeavy Shields and fill main tank role at expense of damage output.
RK's, the lorebreakers (according to forums) out dps Hunters, ive soloed Elites on my low leveller, Can heal reasonably well (they use mostly HOTS rather than a Minstrels reactive heals) some people just plain wont group with them due to them being arses. Has a pet rock.
Wardens, the squishiest tank class, Medium armour tankers. Dps is about par with captains, but their survivability depends on the user. Again a good warden is a site to behold can heal the party whilst maintaining agro on a group but a bad warden is a liability. The gambit system is fun but only if your memory is good so you can go red, green,green,yellow etc and remember which combos do what.
Minstrels are slow to level post 40 as well though not as slow as guardians. Main healers. A few nice utility abilities which are very situational.