You're much better off changing tires based on wear/damage rather than mileage since conditions and tires vary so much. Generally speaking, if they look OK, they are. If you see big cuts in them, they are very visibly worn, or you frequently experience flats, it's a sign that you need a new set.

The best ones to buy depend on what kind of riding you like to do. Most people do well with a decent training tire. I've got a set of Conti GP 4 Seasons on right now, but they don't blow me away. Still not a bad tire