My best Level 1-85 leveling guide

Level 10-20

Westfall (Alliance)
Bloodmyst Isle (Alliance)
Darkshore (Alliance)
Loch Modan (Alliance)
Azshara (Horde)
Ghostlands (Horde)
Northern Barrens (Horde)
Silverpine Forest (Horde)
Ruins of Gilneas (requires level 14, Horde and Alliance)
Redridge Mountains (requires level 15, Alliance)
After reaching level 20, more options open up. From this point on, zones are no longer for Horde or Alliance only. Questgivers can also help point players to the next area. Often times, this is around the time when players start considering the best WoW leveling guide to help them breeze through these levels.

Level 20-25

Ashenvale
Duskwood
Wetlands
Hillsbrad Foothills
Level 25-30

Northern Stranglethorn
Arathi Highlands
Stonetalon Mountains
Level 30-35

Desolace
Hinterlands
Cape of Stranglethorn
Southern Barrens
Level 35-40

Feralas
Western Plaguelands
Dustwallow Marsh
Level 40-45

Thousand Needles
Eastern Plaguelands
Level 45-50

Badlands
Felwood
Tanaris
Searing Gorge (requires level 48)
Level 50-60

Burning Steppes
Winterspring
Un'Goro Crater
Swamp of Sorrows (requires level 52)
Blasted Lands (requires level 54)
Scarlet Enclave (Death Knight starting place, Death Knights only)
Deadwind Pass (requires level 55)
Moonglade (requires level 55)
Silithus (requires level 55)
When considering the areas that go all the way to level 60, it is important to remember that at level 58, players can start questing in Outland, which requires the Burning Crusade expansion.Level 58-70

Hellfire Peninsula
Zangarmarsh (requires level 60)
Terokkar Forest (requires level 62)
Nagrand (requires level 64)
Blade's Edge Mountains (requires level 65)
Netherstorm (requires level 67)
Shadowmoon Valley (requires level 67)
Isle of Quel'Danas (requires level 68)
Once at level 68, players can access Northrend with the Wrath of the Lich King expansion, which opens up the following zones:

Level 68-80

Howling Fjord
Borean Tundra
Dragonblight (requires level 71)
Grizzly Hills (requires level 73)
Zul'Drak (requires level 74)
Scholazar Basin (requires level 76)
Crystalsong Forest (requires level 77)
Icecrown (requires level 77)
Wintergrasp (requires level 77, PvP area that will cause you to be flagged)
At 80, the Cataclysm expansion will allow players to visit these following areas:
Level 80-85

Mount Hyjal
Vashj'ir (this area is underwater, which sometimes complicates things)
Deepholm (requires level 82)
Uldum (requires level 83)
Twilight Highlands (requires level 84)