SOUTHWEST!!! We fly California to Ohio about 4 times per year, and it's just the best hands-down! Festival seating means you can pick your own seat, and if the plane isn't full, that means you get an extra seat to move around in. Add to that no baggage fees, friendly flight attendants, family boarding between A & B group (if you're an A, then you don't have to worry; check in 24 hours before your flight), quick & easy gate check, and liberal baggage requirements (50lb, 2 free bags per flier), and you have one great airline! We don't fly ANYONE else! No one else stacks up!
We've never used a car seat on the plane, though, because both sets of grandparents have their own for our kids (because we visit so often). I understand the safety issues, but we don't drive around all that much, so car seats for us = screaming matches. My 4yr old knows the rules, and I've never had a problem with me jumping around, wanting to get out of his seat, etc. THe flight attendants are really good about kids usually, and we always have a great flight :)