I'd say best at Domestic Coach service.

It is not fair to compare WN to Singapore since they don't fly internationally. Once upon a time, I was a loyal UA flyer. When it came time to go internationally however, I went with AA and its partners. I couldn't justify the extra cost of going UA and one other time, UA and its partners had no seats available on very short notice, full fare, business class.

For WN vs. TED vs. Frontier on domestic routes, Southwest wins without question, in my mind.

If you can get a free upgrade to first class on a legacy carrier, now you've got an interesting proposition and room for debate (Probability of upgrade, cost of seat, change fees, number of times you "pay your dues" on a TED flight for the hope of an upgrade on some other flight, ability to use miles to far away lands, etc.) Also, consider the ability to buy your way up to first class for $50 per 500 miles at USAir vs. chasing status. If you are not flying that route often, that isn't a bad deal.