In WoW Classic, leveling is a bit harder. There isn’t the expectation that you’ll be leveling a couple of times in each zone and then moving on, and some areas are better than others for leveling.
Each race is stuck in their starting zone for a bit no matter what, so you don’t have much choice there. This isn’t so bad for Humans and Night Elves, but Dwarves and Gnomes should get out of Kharanos as soon as they can.
World of Warcraft Classic: Best Alliance Leveling Zones
Once out of your starting zone and the area immediately outside of it, you have some options. In order: Darkshore (11-19), Loch Modan (10-18), Westfall (9-18), and Duskwood (10-30) are all solid starting points.
Keep in mind those level ranges. Duskwood is an excellent zone, but going there straight at level 10 can lead to issues with losing time dying to high-level mobs you accidentally wander into.
Westfall is a key area as it contains the Deadmines, one of the first dungeons you can get access to. The quests are relatively gentle in the area as well, making it an easy area to clear.
Loch Modan is a fast and furious zone full of efficient “kill these” quests you can complete multiple of at a time with very dense spawns. As long as you’re careful not to get mobbed, it’s easy to get your levels and get out of here. Be careful not to wander into the Badlands (36-45) in the south and get killed by the high-level enemies or Horde players there.
Darkshore is much the same, if ever so slightly higher in average level. The quests are even thicker on the ground here, making it the best zone for this level range.
Once you’ve blasted through the content in most or all of those areas, you should be ready to tackle Redridge Mountains (15-25), which should carry you to the point you can finish Darkshore’s higher level quests as well.
By that point, the Wetlands (20-30) is pretty much the only place you can go next. There aren’t a ton of quests here, and most are clustered near the east end of the map. The area is meant to be one last Alliance leveling area before you’re forced to venture off into a Contested Zone…which is a bit of a different ballgame. Just make sure the one you venture into first isn’t the Burning Steppes (50-59) to the north; you at freshly 30 stands no chance in that area.
Keep these zones in mind, and you’ll blaze through the low levels: and be ready to take on Contested Zones.