My favorite teacher from high school, Mr. Bulak, says that there are four kinds of academic papers. The best is short and good, and the second best is long and good. Third best is short and bad, because while it's bad, at least it's over with quickly. And the one you don't want to write is long and bad.
In that spirit, I think there are four kinds of horses. One category is not-so-athletic and extremely rideable. These are your wonderful beginner-type horses, the kind fellows that might be "5" movers on a very good day but are generous in their characters such that riders can learn from them. These are the kinds of horses that, in addition to being able to go to first level, test 1, at the local dressage show, can give pony rides to your kids, go on a trail ride with your spouse, and do the costume class, the Prix Caprilli class and the trail class at the Halloween Fun Show at the barn down the street. These horses may not be expensive, but they are worth their weight in gold.
The second type is extremely athletic and extremely rideable. These are your big-money amateur horses, the fancy warmbloods who do Prix St. Georges but also let you take them on a trail ride; the schoolmasters. This category also contains the well-bred and well-coiffed young horse that, for whatever reason, doesn't go through the charming teenage nonsense that most horses go through, or if he does, it's the kind of bad behavior you fix with a waggled finger, not with detention and a dunce cap. These horses are plenty expensive but are a treat to ride. Ella is one of these—her young horse nonsense was never ultimately all that nonsensical, and now she's a pleasure every day.
The third type is extremely athletic and not-so-rideable. These tend to be professional's horses, the fabulous movers that are also a little (or maybe a lot) naughty. Virtually all young horses, like virtually all young people, at some point, push the boundaries and need a time-out, and many grow out of that stage. Some, however, do not. Billy, at 21, will still jerk a timid rider around a bit, being pushy and sensitive. As wonderful to watch as the top Olympic horses are, I bet that many, if not most, can be real screwballs; there are plenty more that torture their people because they're such great athletes, but so unreliable to compete. It's that thin-line-between-genius-and-madness thing.
The last category is the kind you really don't want: not-so-athletic and not-so-rideable. These are the ones that buck/rear/bolt, and that when you try to train them not to, you find that they're crappy movers or terminally stiff in the back. These are really, really tough horses, because you could get really hurt trying to make them into a useful creature, but there's no money in it; they're not financially worth the risk and the time. I'm very lucky to have never owned one of these!
Like English papers, good editing/training can move horses from one category to another. Good dressage training can take a mediocre horse and turn him into something special. Maybe there's no way to train an OTTB or a draft-quarter horse-wombat-alpaca cross to be the next Valegro, but there are lots of "normal" athletes who've learned a little sideways and a clean flying change to take someone on a budget to third level. Oodles and oodles of phenomenal and screwy young horses grow up into phenomenal and dependable FEI horses; Midgey was a royal squirrel as a kid, and now I'd trust just about anyone on him.
Unlike English papers, there's no "best" category (though I think we can all get behind that last category being the worst). You try and find the horse that's right for the job that also fits into your budget, and you do everything possible to get the most help from the best trainer you can, and you ride that wave as far as you can.