Lyons Classic Pinball should be #1 by a long shot. Open Thurs-Sun nights. Thurs is league night, but that only occupies 5 of the 35+ machines - so it's easy to go and play even if you aren't in league. And there will be good people to meet (like me!).
1-Up and 1-Up Colfax also have great selections, the games are in fantastic shape, and they are open more often. I never interact with the bartenders, so I have no opinion. However, if it's a Saturday night (possibly also applies to Friday and Thursday), you might want to avoid 1-Up which will be jam packed with screaming drunk people. Colfax is probably better on one of those nights, but I haven't been (at least there is more open floor space and the pins aren't 10 feet from the bar).
Press Play (in Boulder): meh. Used to have 9 pins, replaced 5 recently with air hockey. Condition depends how recently they've been serviced, which is not always that often. Pinball is definitely an afterthought there, though they're in a separate room so it's still okay to play on a busy drinking night (which will be college kids who mostly don't care about the games).
Backcountry Pizza (in Boulder): I was there last week and it was just Tron, AC/DC, and Genie (with X-Men turned off). Games play decent and are in a separate room.
Tilt (in Louisville): This place is probably good for very casual players. Not good if you're serious. I went about a month after they opened, and about half of the machines had issues (not level, plays slow, slingshots not working, coin mechs eating quarters, balls getting stuck). A friend went a few weeks ago and said that pretty much every machine has issues now. The atmosphere is more like a YMCA than a place to play pinball unfortunately.
Pinball Jones (in Fort Collins): Never been, but I hear good things.