I use and love these 1.25", 90 psi tires on my MTB:
http://www.performancebike.com/shop/...e.cfm?SKU=2314
(tubes:
http://www.performancebike.com/shop/...e.cfm?SKU=1446 )
I'm 148 pounds and my MTB weighs 22.5 pounds...heavier riders may need to go with 1.50" tires.