Denim is my obsession. =] I've had really good luck with J brand jeans, (specifically the 914 cigarette, and their skinnies). I buy them just a little snug (size down if they 2% spandex or more) and by the time I have broken them in for like an hour they fit perfectly and don't stretch out anymore!
In general I find, with a lot of designer jeans I have, that if they are made of anything but 100% cotton they will stretch out a little after you wear them for more than an hour. So I always make sure they are a little snug (not uncomfortable) in the fitting room- that way they are perfect throughout the day.
It really does depend on the material/designer and DEF the cut/style. Some other brands that keep their shape (IMO) are J brand, Adriano Goldshmeid, Hudson Jeans, Paige, Current Elliott, and some citizens (depending on the style). True religions, Joes and william rasts tend to stretch out the most (for me) out of all my designer jeans.
Just bought some dylan george skinnies today- so we will see how they turn out! -Never tried them before!
I know nothing about brands, but my time as a weird-shaped consumer and my time in retail has taught me that if you have no *** and find your jeans sagging where your *** is supposed to be, get jeans with a bit of polyester and spandex in them to help them keep their shape.