As a long-time instructor who has worked at all of these areas (Breckenridge, Keystone, Arapahoe Basin, Vail, and currently Copper), I can tell you that you can find great skiing for pretty much anyone at any of them. Each has its own personality and quirks, and you'll certainly get the most out of any of them with a knowledgeable local guide or instructor.
Loveland and Arapahoe Basin are considerably smaller than the major resorts of Keystone, Breck, Copper, and Vail, but they can knock your socks off on a good day. There's no lodging or dining beyond the typical cafeteria fare, so people go there for one reason only: to ski. Most of the lifts are slow fixed-grip chairs. That's not necessarily a bad thing--they keep the people who seek just quantity away, and give you a good restful break between runs on some truly awesome and generally uncrowded terrain--at least on week days. Both can draw quite a crowd on weekends, so if you go, get there early for a good parking place. Both are legends, where the raw, unadulterated spirit of skiing remains alive and well. Go there with respect!
To describe Breckenridge as "flat" is to not know Breckenridge very well. It has some of the best steeps in Colorado, between the terrain off the T-Bar (Horseshoe Bowl), the upper terrain on Peak 7, the Lake Chutes and other terrain from the Imperial Express chair, and the glades and bumps off E lift, among others. There's no doubt that it helps to have a guide (and the ski school is the home of a bunch of top pros), but even without one, the upgraded lift system over the past several years has made getting around the mountains of Breckenridge much easier and more intuitive. The biggest problem with Breckenridge, as others have noted, is parking. I'd plan to bite the bullet and park in the pay lot at the new Gondola. Another very good option is to take the free Summit Stage bus and avoid the problem altogether. From Dillon, your best bet might be to drive to the main transfer center in Frisco (behind Walmart) and take the direct bus to Breckenridge. The other common complaint about Breckenridge can be the weather--it's notorious for wind.
Keystone is perhaps the most under-rated resort in Summit County. Although it has had some pretty good terrain for nearly thirty years since the opening of North Peak, it has had a hard time shedding its early reputation as a beginner and intermediate resort. The front side of Keystone is nearly all groomed green and blue runs--and as others have said, they can get extremely crowded with over-zealous, under-skilled skiers and boarders. But North Peak and The Outback add some very good bumps and--especially--open glades and tree runs. The lift lines can get long, but the ungroomed runs, at least, are rarely crowded. Nothing there is breathtakingly steep, but there really is some good skiing to be found at Keystone, if you know where to look. Keystone also has the only night-skiing in the region. It's an acquired taste, but with the new gondola and the warmth of the Summit House, along with a pair of clear-lensed goggles, it can be magic. Keystone is surely the most sheltered of Summit County's resorts, as well as the lowest in elevation (although with a base elevation of over 9300', it certainly is not low)--making it a good bet when the howling Rocky Mountain winds kick up. Parking is OK--free in the huge lot at the River Run (gondola) base, although you do have to walk all the way through the village to get to the lifts. Unfortunately, the Summit Stage bus route to Keystone meanders all over the place and takes seemingly forever from just about anywhere, so I probably wouldn't bother with it. Keystone's biggest drawback tends to be significantly less snowfall than any other resort in Colorado, although this season, at least, it has had great snow and the conditions should be more than fine. When Keystone does get a good dump, the (not inexpensive, unfortunately) snowcat service can get you into some of the best powder skiing in Summit County.
Copper Mountain is the perennial locals' favorite in Summit County, and for good reason. It has a little of everything, and a whole lot of most things. The groomed runs on the front side are some of the longest and most consistent--and least crowded (although they can get busy)--anywhere. You'll find long bump runs off the Alpine Lift on the east side, awesome steeps with long-lasting powder on Union Peak, Spaulding Bowl, Copper Bowl, and (via the free snow cat) Tucker Mountain, as well as great trees, astonishing views, and Copper's renowned "naturally separated terrain" that provides the best easy learning runs for beginners, totally segregated from the speeding advanced skiers elsewhere on the mountain. Copper Bowl offers a great variety of off-piste steeps, although its southern exposure can make for challenging conditions sometimes, particularly in the morning following long sunny spring days. Parking is reasonable, with free-parking in the huge outlying Alpine Lot at the east end of the resort, and a reliable, free bus shuttle service to the rest of the resort. There are also pay lots within the resort, within reasonable walking distance from the lifts. And the Summit Stage is very convenient from Frisco. With new ownership, Copper Mountain anticipates considerable upgrades to lifts and other facilities in the next few seasons, but currently the lifts are a mix of modern high-speed quads and a six-passenger chair, along with a few old fixed-grip chairs. One high-speed Poma (platter-pull surface lift) runs along the exposed Hallelujah Ridge to serve Spaulding Bowl and other steeps to the west and south--worth riding (for advanced skiers) in all but the worst weather. Perhaps Copper's weakest point, for some, is the relative lack of on-mountain dining choices, but the cafeterias in the base areas are above average--especially (in my opinion) Copper Station in the East Village, with its many daily $5.99 specials (you can't even buy a plate of french fries for that in some Colorado resorts). On the mountain, the only indoor lunch spot is Solitude Station, mid-mountain at the top of the American Eagle chair. All-in-all, I think you'll have a great time at Copper Mountain. Lift lines are rare, although not entirely unheard of on busy weekends and holidays--especially in the Center Village (Eagle and Flyer lifts), and the Poma line can take a while when the weather is good.
Copper Mountain and Breckenridge typically--but not always--get the most snow in Summit County. Breck, Loveland, and Arapahoe Basin get the most wind and Loveland and A-Basin--with so much above-treeline terrain--can get especially inhospitable when the weather turns severe.
They are all first-rate ski areas. Enjoy!