31.
And, yeah, I know but, where is a good place to go that isn't garbage? The next area Arathi Highlands is supposedly garbage. I did a few quests and they kept sending me all over the map...and it wasn't interesting quest either.
In general the zones are going to get more spread out and less focused until 60, at which point you're going to hit BC content, which is basically the oldest leveling content in the game and it shows. When you get to 80, you'll be back to zones more similar to what you've been doing and it generally gets better from that point until 110.
Since you're 31 you can safely skip Arathi. Hinterlands is a 30-35 zone just north of Arathi, and I'd look for it in your adventure guide. It's been some time since I've done the Horde quests in that zone, but it's always been one of my favorites. And it will allow you an easy transition to Western Plaguelands (35-40) and Eastern Plaguelands (40-45), which are good zones, IMO. Also allows an easy way to get to Badlands (44-48) from Eastern Plaguelands, which has one of the community's favorite quests in the game.
Your other options for 30-35 are Cape of Stranglethorn (if you don't like Arathi's spread out quests, I don't think this is a good option for you), Southern Barrens (great zone, and gets you on the path to Dustwallow Marsh and then Thousand Needles, one of my favorite zones) and Desolace (barely remember anything about the current iteration).
Southern Barrens and Desolace are on the other continent, Kalimdor, and I don't know how easily you could get to them. Stranglethorn is on your continent, but the easiest way there is to take a zeppelin from the Undercity to Org, and then from Org to Stranglethorn.
So my suggestion would be Hinterlands, and if you don't want to do that, Southern Barrens.