The Warrens ~ (4-12)
Netherbian Lair ~ (15-30)
Echo Caverns ~ (20-40)
The Scarlet Desert ~ (30-45)
The Tower of Frozen Shadow ~ (30-45)
The Hole ~ (40-60)
The Grey ~ (40-50)
Howling Stones ~ (50-60)
The Halls of Betrayal (Chardok B) ~ (50-60)
These were the hotspots around April... dont know if they have updated since then... BTW, this was from Infernals post:
http://www.redguides.com/community/showthread.php?t=631
Hope that helps a bit