I try not to fly anywhere that Southwest doesn't fly. Service on the other majors is always bad.
Agreed, however as a volume based carrier they don't usually fly to the places like MT, AK or Canada. Denver being a hub for them I always check them first though. Flying to Winnipeg for a fishing trip this year and not looking forward to ridiculous bag fees and terrible on-time rates on United or Delta.
Another shout-out to Kayak, however most fares are so coordinated anymore so usually they are all within a couple dollars of each other. The only real deals are email-based time sensitive ones. For example: opening day of baseball Frontier had $1 flights to Vegas from Denver, but only if you were on their email list.
I try not to fly anywhere that Southwest doesn't fly. Service on the other majors is always bad.