I was farming in Netherstorm the other day when I heard an ongoing complaint-fest in general chat about the Consortium grind. It was "too long", "annoying" and "boring." Since the patch, I've heard the same thing about the Sha'tari Skyguard quests ("it's like Timbermaw all over again!")
Personally, I don't mind rep grinds -- they give you something to do solo, after all. But some are better than others. The easiest rep grind I've done is Sporeggar, by far. Since no one ever does it, they just put their Fertile Spores and plant fronds on the Auction house, where I can pick them up cheap. The whole thing was basically an afterthought, and that Primal Earth to Water transmute has been one of my most consistent moneymakers as an alchemist. The Shrouding Potion has the potential to be interesting, too.
Worst? Well, I never did Timbermaw all the way, and as Horde, I haven't had the opportunity to do the Wintersaber Trainers, a rep grind apparently so painful that it has its own support group. But I'll have to say Cenarion Circle. I only did that up to revered, and I will still say that I never want to see another fricking silithid or cultist as long as I live. I still have the piece for the AQ20 dagger in my bank, left unfinished just because of my pure hatred for this faction. Stupid hippie night elves and tauren ...
What's your favorite and least favorite rep grind? Which factions do you think could use better rewards? Should faction rewards be competitive with items from raids and dungeons?