The first bodycon dress I owned came from a market stall, cost next to nothing, and taught me that cheap stretch fabric has a memory of its own. By the end of one evening it had dropped two inches and bagged at the knees like old tights. I learned that stretch which doesn't recover is worse than no stretch at all.
What separates a good one from a disappointing one comes down to the fabric holding its shape after it's been sat in, bent in, danced in. A proper one has real weight to it, a decent percentage of elastane mixed into something sturdier than thin jersey, so it skims rather than clings in the wrong places and springs back when you stand up instead of pooling at the backs of your knees. Seams matter more than people expect on something this simple. Flat seams at the sides and a lined panel through the middle stop the dress riding up when you sit down, and a proper hem that doesn't curl after one wash tells you the whole thing was made with some care. Cheap ones give themselves away at the zip, usually a thin metal one that catches, or no zip at all, just stretch and hope, which is fine until you need to actually get it off at the end of the night. If you're starting from nothing, go for a midi length in a thicker double knit rather than a mini in thin jersey. The longer length is more forgiving round the stomach and hips, it doesn't need constant tugging down, and it photographs and moves better on most body shapes, whether you're pear shaped with more to carry on the hip or straighter through the middle. A good midi bodycon sits at the natural waist without digging and skims over the stomach rather than outlining every seam of your underwear, which the bad ones do without fail.
On colour, black is the easy starting point because it hides the clingy bits and goes with anything you'd throw over it, but a deep bottle green or a rich burgundy does the same forgiving job while looking a bit more considered, and neither shows every crease the way pale colours do under certain lighting. Nude tones and bright white are the ones to be wary of until you know the fabric well, because any seam or pull shows up instantly.
This is the dress for anyone who wants to feel pulled together without needing to think about an outfit, from twenty somethings heading out straight from work to women in their fifties who want something that skims rather than shouts.