Thick Leather by far the best spot is BRD:
http://www.wow-professions.com/farming/thick-leather-farming
I've done that Badlands spot before and it isn't nearly as good (stupid hilly terrain is stupid). Just make sure you're not dumb like me and use a pet/totem to kill the rounded up dogs, since if you don't tag them yourself they'll rapidly despawn and you won't be able to skin them. Nets ~50 leather a run.
For Rugged Leather, best by far is Onyxia's Lair. Just kill the 4 trash dudes in there and reset. Goes really quick. 30-40 leather or so per run (though they give you hides instead frequently enough to be annoying). Note that you do need to have 435 skinning here which is higher than it would take to farm Rugged Leather elsewhere. But oh, so worth it.
Knothide Leather I did Karazhan as described here:
http://www.wow-professions.com/farming/knothide-leather-farming. Pretty damn fast. Not sure how the other spots hold up - didn't need to go to Outland at all besides for the Fel Scales needed.
Borean Leather I'm not sure - ended up buying most of it. The wow-professions site recommends Gundrak but I didn't get to try it out. Not the longest grind anyway no matter where you farm.
For Savage Leather - the last grindy one (you need like 500+), there's Zul'Gurub which is supposed to be good, but I didn't try that out since my guy isn't very geared (and I would have had to get myself over there - maybe if you're Alliance it's more convenient.) Anyway, I mostly followed
this comment's method after getting frustrated with how slow it was at Tol Barad and elsewhere. Ended up being pretty fast, even when having issues with the crocs. There's also
this way which I didn't try because I was going to be dropping skinning anyway and didn't want to drop the effort into getting the quest chain.