Here's some overland areas, as I'm not a big fan of dungeons. Must be all that bad pet pathing...
30-38
Enchanted Lands
Zek, the Orcish Wastes
38-45
Feerott
40-48
Everfrost
Lavastorm
48+
Desert of Flames
Within each of these areas are special dungeons, instances, or mobs that are harder than the level range. Generally, the further and deeper into a zone, the harder the mobs.
Also look at Splitpaw Saga in Thundering Steppes. It is a scalable zone meaning the mobs will be your level range when you fight them.
Bluie is right, the best exp and loot comes from group encounters and heroic mobs. And the only way you can defeat those are in a group. Group encounters are found in all areas of the zones, you just need to look for them. Believe me, it won't take long to find them. :)