If you don't mind not having an assigned seat #, Southwest is the way to go. There is a trick to getting a good seat on your Southwest flight or flights though, early check in. For those not familiar with the boarding process for Southwest, here's a run down. Southwest boards it's flights by groups and then numbers within the group. There is a group A, B, C, D, etc. depending on the number of seats booked for the flight. Each group then has at least 40 spaces within them. The earlier you check in for your flight, the more likely you will be in group A. The trick to this is to purchase early check in when booking your flight. My fee for this was $15. The airline checks you in automatically up to 36 hours before your flight. On my most recent flight, I did not wait for the airline to check me in. I checked in from home and printed my boarding pass, 40 hours before the boarding time, I ended up with a boarding position of A 22. You see, at the gate, 30 minutes before flight's departure time, the agent will begin calling out the groups starting with A. Passengers line up by their assigned group and their number within the group. Seats are first come first served once on the plane, so a person with a group A 1 position basically has the whole plane to choose a seat from. So if you wait until the day of and the last hour before your flight boards to check in, you can be assured you will be amongst the very last group boarding your plane.
Another reason I like Southwest is their baggage policy, your first checked bag is free and carry ons, limit of two per checked person, are free also. Sure the fare for your flight on Southwest may and I say may, be about $25 more than other carriers on same route, the good service and reputation of Southwest Airlines makes up for the difference. In addition to this, most Southwest flights have free wifi, and for me because I love their snacks, an unlimited supply of snacks & nonalcoholic beverages. On this flight, three separate offerings of snacks & drinks were served. Each time I was allowed to get as many of each I wanted. If I wanted any of these in between the serving times, I had no problem getting what I wanted.
On this flight our plane had to either detour around or over weather on five occasions. The pilot kept us informed well in advance of any of these changes and did his best to keep the flight smooth during each of those episodes. The cabin attendants were, and have been on every other Southwest flight I've ever flown, very polite and caring too.