Ive got to say, thats a tough question, and highly subjective I guess. Ive been working on my Pally and Warrior lately and I whined about warriors being a drag, but I admit it got much better once I got smart and specced Arms, and also as got higher level. Paladin has some cool buffs and its so so nice to not have to bandage and eat every fight. When all else fails - just bubble and run for your life. The rest of my alts or too low to really give an educated opinion, so Ill spare you. Of course, my main is a hunter and I love it. Im my own 1 man gang with Winnie the tank or PvP killer with Nikki and her lightning fast claws.
As for instances - Hunters are the most underrated class in my opinion and always get a bad rap. The versatility and fun of being a hunter may be the cause of that, as many people roll hunters for the wrong reason, and never master their skills as they should, therfore not contributing much to groups, and maybe even causing wipes by not controling their pets correctly. A common mistake by hunters and non hunters alike, is simply dismissing the pet, and losing the benefit of having it active. The pet is a valuable addition to dps, cc, off tank, and effectively makes a 5-man instance a 6-man run, especially with BM builds.
I can lay a freeze trap, Misdirect a group to a tank from 41 yards away, send my bear to off tank 1 mob, and use Distracting Shot on another to bring him to my trap. Keeping 2 mobs busy while taking out main target. And if the Sh@t hits the fan, I can stand to take a few heavy blows, and kite a mob all over town if needed.
Ive been all builds and had many pets. So far Beast Mastery is the most verstile and most fun, even at the expense of some (barely any) dps. I've got:
Great dps, even with BM build - but also a aggro reset thats best in the game, so I can dish out the damage without pulling aggro off tanks.
A little under 8k hp unbuffed both for me and pet.
Almost 7k mana pool.
Mail armor, which means I can take some serious hits and still survive, unlike the high dps but sissy dps casters.
41 yard range with nice burst damage.
Spirit Bond - 150 hp gain for me and pet every 10 seconds
Can use almost any weapon, duel wield or 2 handed.
Wing Clip - slow a target, giving me time to get out of melee range and put on the big hits.
Aspect of the pack - increase party run speed great for saving time indoors where you cant mount.
Animal Handler - 8% speed increase while mounted - Awsome perk
Freezing trap - Underrated CC, I can chain freeze a mob in combat, basically forever, with my Beast Lord bonus I get a 4 sec reduction on CD and its even more effective than ever.
Ferocious Inspiration - Every time my pet crits (which is very often), all party members get a 3% increase in damage for 10 seconds.
Intimidation - Next pet attack causes a high amount of threat and stuns the target for 3 seconds.
My favorite - Bestial Wrath and The Beast Within - My pet and I go into a rage (besides awsome graphics) and cause extra damage, and best of all - "Do not feel pity or remorse or fear, and cannot be stopped unless killed" I love it 
Misdirect - Awsome abitily that makes a hunter even better geared for pulling then was before - It directs the threat of my next 3 attacks to a target of my choice. Can be used in combat.
So that was my "Hunter is the best" rant. I hope its not just spam. Maybe it will do something in the way of getting other classes to know how to take advantage and also to appreciate having a hunter by their side.
I'd love to hear some other opinions, maybe ill roll a casterm, even tho its not really my cup of tea.
not that easy 
