Can't speak for it, but I do understand that many make you go to certain gas stations only. I have the same issue with the gas card for my work van.
With that said being in Colorado I would go for the Conoco/Phillips 66 credit card since you have plenty of them across the state(I know because I go there most of the time) and if used in conjunction with the Fuel Forward app(used to be Conoco and Phillips 66 app) you get $0.15 off at the pump every fill up. That said you get the same thing with direct pay as well.
I looked into it just to see the offerings and it seems fairly low/middling at a flat .03 cents per gallon rewards. I know there are bonuses and whatnot but you've got to really ignore those and look at your base rewards.
For example my Wells Fargo Active cash - which has a flat 2% return on all purchases, regardless of category, will return me a little more than 9 cents per gallon at my local gas prices. When I factor my local fuel rewards I'm saving between 15 and 35 cents per gallon on average with every fuel up and I'm satisfied with that a constant reward.
If you want a gas card specifically, I highly recommend Nerd Wallet. They seem to have really good intel insofar as credit card rewards and "Bang for your Buck"