The fastest way to level is through PL (power leveling). Unfortunately this takes away from the point of the game. Also, it is hard to find someone to do it (either busy, crafting, or quitting etc.). The following areas are suggested. This applies for Midgard and "BG" indicates a battleground.

Level 1 through 5: Ft. Altla, Galpan, and Aheirs Landing
Level 6 through 7: Lions Den (BG), farther out from Ft. Atla
Level 8 through 11: Nisse's Layer (with strong group)
Level 12 through 14: Nisse's Layer (group),Weeping Willows by Gna Faste
Level 15 through 19: Deep Nisse (group)
Level 20 through 22: The Cursed Tomb (group recommended)
Level 23 through 25: Thidranki (BG)
Level 26 through 29: Vendo's Canvern
Level 30 through 32: Deep Vendo Cavern
Level 33 through 34: Wilton (BG)
Level 35 through 36: Skonka Ravine, West Skona, Varu
Level 37 through 39: Molvik Faste (BG), Deep Varu
Level 40 through 42: Moderngav (the place you get faction), and River Of Death
Level 43 through 45: Levik (BG)
Level 46 through 47: Deep Moderngav (very strong group in frontier)
Level 48 through 50: Frontier, PoC, massacring level 50 albs, and terminating level 50 hibs