United is my favorite airlines for award redemption - I redeemed more than 500,000 miles in the last twelve months for destinations ranging from 48 states, Hawaii to Asia. Best part is that UA doesn't charge junk fees such as rush service etc.

I also had good experience redeeming miles for Europe travel.

Delta is OK for domestic awards, however, I don't like the fees they charge.

Northwest is terrible in award availability - I wanted to spend down my workperks balance but I simply couldn't.