Breck is the best for beginners, but much pricier. You can live on the quicksilver lift and there's like 3-4 runs over there that are maybe the easiest runs in all of Summit County to learn on (they are pretty boring for advanced skiers though, I usually try to ski them backwards if I can LOL). The chair is huge and it's one of those super slow chairs that loads really really easy.
Loveland isn't too bad. Vail/BC aren't awful (but Vail is weird because all the easy greens are at the top of the mountain).
As someone who has taken beginners skiing a ton, I can't disagree MORE with Keystone, it's the LAST place I'd take someone who's never skied before. Keystone is great once you get past the initial beginner stuff, but you really only have one small run for beginners and that's SCOUT with a tiny 2 person slow chair to get back up (which is steeper than the runs off QuickSilver @ Breck).
Talk to any of the instructors at Keystone (I know a few of them) and they said all the time the worst thing that happens is that people who have never skied before attempt schoolmarm. It's just too dang long and once you're on it, there's no easy way out. If you're day 3 and really getting your skis under you, schoolmarm can be good. But for day 1 NO WAY, day 2 would only be for the athletic types that learn fast.
I ski on average 20ish days a year for the last 15 years, but I'm not a local and I've brought several people who are brand new skiers to different mountains in CO.