I use Purdy brand roller covers. The shorter the nap generally the smoother the finish. Using oil based paints it is usually easier just to throw it out. The costs, mess and environmental impact of using solvents to clean a roller cover with generally cost more than the cover itself.
For latex / water based I use a roller cleaner. The best one I have used is an attachment to a hose. It is a plastic ring type thing sort of like this one. Attach a hose, put the roller in it, turn on the water and push it back and forth. It's clean in a minute. It makes a titanic mess so do it in a tub or outdoors where the runoff won't kill grass. latex paint kills grass too.