I would recommend getting an airline credit card for what every airline you pick
1. United is the top pick for an Airline in Denver -- but the hardest to get status with I have Silver via Marriott but will lose that at the end year they fly out of B terminal and are the largest Carrier at Denver. If you fly enough and the your company is paying for your flights this is your default pick. They have 2 lounges in B terminal (East and West). I own my own business so I am too cheap to flight United much. I currently have the United Club Card but will be down grading it to the Explorer card in 2019.
2. Southwest is my Primary airline I keep status with them (A list) they fly out of terminal C--2nd largest Denver airline
3. Frontier I used to have status with them and I have going to give them another try in 2019. I will only use them if I have status with them to get their stretch seating. You can get status with their credit card spending of $20,000 and up on their card. They fly out of A
4. New Amex Centurion lounge is coming to Denver terminal C in 2019. I will get Amex Platinum card in 2019.
5. New premium plaza lounge is coming to Denver Terminal A in 2019--this should be Priority pass lounge.
6. Currently you can dine at Timberline restaurant with priority pass
7. Delta has a lounge in Terminal A opened 2016
8. AA and BA have a joint lounge in Terminal A