IMO, these are the classes you should go for (ranked by which you should attack first) and the classes you should stay away from (ranked by which class you should avoid the most).
Go for:
1. Rogues - their dodge procs our overpower (which if specced correctly in your arms talent tree, is essentially a free crit). Over the 29 bracket (when twinked rogues are the norm), they're the butter to our knife [exceptions can occur at lvl 70, but are rare].
2. Warriors - they have the same moves as you, so they can't pull anything you don't already do; very gear-dependent, if you trust your gear, you shouldn't have a problem.
3. Druid - druids can do great dps, but they're usually better off in 1v1 fights; if you can attack them in a group, most druid players will get confused and you'll have an easy time pounding on them.
4. Shammy - shammies can do great dps, but they're usually better off when they have time to pop totems; if you attack them on the move, you have a good chance of killing them.
Avoid:
1. Hunters - a good hunter can easily kite while doing constant dps, it's very hard to get into attack range; you also have to be careful of charge/intercept, as it's a direct line to them (easy for them to lay traps). Really only possible after you have all stances and a lot of experience with stance-changes/ rage spending.
2. Ice Mages - their aoe freeze has the same cd as your intercept, but they also have blink. Best to wait until after they've popped aoe freeze, then attack; try to stay behind them (jump over them at close range).
3. Warlocks - Depending on how good your fear-resist capabilities are; they can also cast while you're behind them. Hit them hard and fast, make sure you're popping that anti-fear rage (also use shout to fear minion); stay away from 42+.
4. Pally - possible if you have good burst damage, but in a 1v1, it's very difficult. We don't have any healing until later on with second wind, so in any balanced fight, the pally will probably win. Possible to kill them at higher levels (40+).
5. Priest - rage starvation and instant fear; don't attack unless you have people helping you, because it'll be a waste of your time.