I just hit Exalted for the 4th time. I am by no means a reputation freak, but I try to get them all as high as possible when I can.
I remember my attempts to grind up my Argent Dawn and Cenarion Circle rep. There were some pretty cool rewards you could get if you were successful, but I just have no interest in killing the same guys over and over for 2 rep points per kill, or collecting items with low drop rates for 50 points per turn-in.
The first faction I ever got to Exalted with was the Brood of Nozdormu. You gain points with them primarily by killing stuff in the Temple of Ahn'Qiraj (aka AQ40), though you can also get a little by killing bosses in the Ruins of Ahn'Qiraj (AQ20). Since I was raiding AQ40 from the time it opened and we eventually managed to clear the place (first guild on our server to clear it) I got a ton of rep points. The reward for that was some very nice gear, and a very nice ring (at the time) that you could turn in for a better one each time your rep increased.
I've always found the way that Reputation works in WoW to be kinda...strange. You would think that everyone in the raid would go directly to Exalted as soon as you kill their main boss. For the Hydraxian Waterlords, that was Ragnaros (who I killed countless times), for the Brood of Nozdormu, that was C'thun. For Cenarion Circle, that was Ossirian. For the Argent Dawn, Kel'thuzad. Sha'tar, Kael'thas. Cenarion Expedition, Vashj. And everyone wants Illidan dead. It's just weird that a faction NPC would say "You've slain our greatest enemy. Never again will the world be subjected to his/her nefarious deeds. Here's 12 gold and 350 reputation points."
Anyway, I digress. Second faction to Exalted was the Sha'tari Skyguard. But that's a faction that is MADE for everyone to be able to easily get to Exalted with. Same with the Netherwing (my third) and Ogri'la (my fourth).
I wasn't even trying to get my Ogri'la rep up. There's nothing I need from them. But the quests are easy, and I've been doing them solely for the cash reward.