Domestically, Southwest. (I really was very impressed with Air Canada, I've flown them twice, and both trips went very well.)
In Europe, my choice would be British Midland for service, though I'm perfectly happy to fly Ryanair when I can go r/t from England to another country for under $10! (The annoying thing about Ryanair is the duty-free sales; the seating practices, etc. are like Southwest, but EVERYTHING, down to the water, is for sale. The flight attendants spend the whole flight trying to sell you perfume, liquor and chocolates. The FA's also act as the ground crew; it's really funny watching women in high heels, berets, and ankle-length double-breasted powder-blue woolen coats pulling the chocks out from under the landing gear wheels.)
Transatlantic, my favorite would actually be Continental these days; they have decent seat pitch and VERY large overhead bins in coach, which means that you can easily clear the area at your feet. The food is decent, too.