For longer rides (75+ miles) I like a 44/18 combo. We've got lots of hills around here, and quite a few 2+ mile long climbs at 5% - 9%. It's a suck combo for long downhill stretches, but I wouldn't want to go much higher because of the climbing.
__________________
"I feel like my world was classier before I found cyclocross."
- Mandi M.