WIP, but this is what ive come up with so far.
Best time from 1-20 on Beta for me at the moment is 2:40 - Without any rares. I feel its a bit slow, need more speed.
Elwyn Forrest: 1-10
Redrige Mountains: 15-20
Northern STV: 25-30
Cape STV: 30-35 (This part is semi-slow, dungeon runs if boosted perhaps?)
Western Plaguelands: 35-40
Eastern Plaguelands: 40-45
Searing Gorge: 48-49/50 (This is dependant on how fast you are lvling - Leave SG ASAP when it gets slow)
Burning steppes: 49/50-52 (Great zone imo - Dont finish it though, later quests are long)
Swamp of Sorrows: 52-54
Blasted Lands: 54-60
Depending on how many lvls you take in the later zones, you may or may not be able to reach 60 in blasted lands. Personally i will be staying in Vanilla untill 60 - its faster than going outland at 58.
Hellfire Peninsula: 60-63 (Last runthrough in HP i made it to 64, doubt ill stay here longer then 62, max 63 this time around though)
Zangaramarsh: 62/63-64/65 (Reaching 65 after HP and Zangra is done is what im aiming for)
Borean Tundra: 68-72
Dragonblight: 72-73 (Hate this zone, leave ASAP upon getting 73)
Grizzly Hills: 73-75/76
Zul Drak: 75/76-77/78
Scholazar Basin: Til 80
Northrend as a whole is my least-favourite leveling place, i try to minimze the amount of zones i will be visiting. If you dont manage to reach 80 in Sholazar, do some starters in Storm Peaks/Icecrown.
Mount Hyjal: 80-82
Uldum: 83-84 (84 and a half, leave when the longer quests come)
Twilight Highlands: 84-85
Cata zones are fast&fun - enjoy them !