Here's the low-down on each denomination:
Cents
Pros: Lots of wheats and canadians, cheap, about 20% copper per box
Cons: No chance of silver, not worth the time to manually roll once done
I've done maybe 15 boxes of cents, and wheats ranged from 1-20 per box. It was so tedious to re-roll them that I ended up tossing all rejects into a bucket and taking them to a coin counter.
Nickels
Pros: Cheap, good chance of silvers and buffalos, easy to re-roll
Cons:
War Nickels are only 35% silver
I did about 50 boxes while I was active (2009-2011), and averaged 1
War Nickel and 1 buffalo per box (Brinks from Bank of America, northwest metro Atlanta), plus about 11-15 "keepers" (1938-1959) per box. I really enjoyed searching nickels, and nearly completed a full set from circulation.
Dimes:
Pros: Better chances of finding silver
Cons: Extremely few "keepers", moderately expensive
I only did one box of dimes (2010?) and got skunked. I found a total of 8 silver dimes while I was a cashier (2008-2011), all of which were from direct circulation.
Quarters
Pros: Good amount of silver per coin found, lots of fun designs to search
Cons: Expensive, silver is almost non-existant
I did one box back in 2010-ish, and found a healthy mix of BU state/territory/ATB quarters, but no silver.
Halves
Pros: Chance to strike it rich, lots of fun NIFC (Not Intended For Circulation)coins to find
Cons: Have to be special-ordered, very low chance of finding anything good.
I did about 4 boxes from 2010-2011, with mixed results. The first box had 4 90% silver coins and a good dozen 40%, plus a couple proofs and NIFCs. The second box had 1 40% and a few other NIFC. Boxes 3 and 4 were skunks.
Large dollars:
Pros: Good chance to find 40% coins
Cons: Good luck finding any, ever.
I think I've seen one
Ike dollar in circulation.
Small dollars:
Pros: Slight chance at NIFC coins
Cons: Expensive as heck, and nothing of real value to be had.
I did one full box of small dollars back in 2011 and found nothing. Plenty of rolls yielded exactly 2 NIFC Sac dollars (2004 and 2005).
So, nickels or cents if you want to have fun, half dollars if you're feeling lucky, and dimes, quarters, or dollars if you hate yourself.