Top 3 domestic already id'd (UA, WN, and F9). Among those, I have flown UA and F9 recently. I'm not quite as negative on F9 as amanuensis is, though my assumption with F9 is that I will need to get The Works package. It is the price with that package I compare to WN prices and UA coach (not basic Y) fares. The last several times I have looked for a flight when WN was a possibility, the fares were at least $100 more each way, and in some cases more than business on UA (or AA or DL). So I can't add anything on WN.
I'm not professing that either UA or F9 is perfect, and I have had more issues with UA than F9 in the last year or two. In one case, about an hour before departure, UA swapped equipment; the original landed and then took off to its new destination. The replacement plane, which had been having maintenance issues for hours before our they swapped, was eventually pulled out of service and the flight cancelled "for maintenance reasons."
The biggest pain with F9 over the last year in DEN has been that they seem to change gates about 5 times before departure (OK, maybe a slight exaggeration). F9 is in the A concourse, so it can be walked from the terminal. For UA (B Concourse) and WN (C Concourse) flights, getting on the train is required except when there are maintenance issues. Not a big deal usually, just FYI.
If you're flying internationally, there are flights from non-stop DEN, but they tend to be more expensive than flights with a connection on the east or west coast. See this page:
https://www.flydenver.com/flights/international_service (some listed are seasonal). If you're going to a particular international location periodically outside North America and want a non-stop flight from DEN, then your options will generally limited to one alliance, except maybe LHR. (And I'm sure our friends on this forum will note if I missed other destinations.) Though when I can, I generally choose flights so that I can clear passport control/customs in DEN instead of other US/CA gateways.
There are 2 UA/*A clubs in Concourse B. There is an AA/OW club en route to Concourse A if you walk; IIRC this club is also available for qualifying pax flying some of the other international carriers that go out of A.
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