Hmm, Young Wizards is the longest and the one I'm most familiar with -- also a fairly easy read since they're YA, though they don't shy away from mature ideas. If you've read TNG:
Dark Mirror, you'll find some familiar concepts in the second YW book
Deep Wizardry. Or you could start with the Feline Wizards trilogy, which is in the same universe but can stand alone and is not YA.
Most of her other original stuff is standalone novels or nearly so, except for the Middle Kingdoms series including the
Tale of the Five, which was supposed to be four novels but has been stalled at three for several decades now, though she's done some other things in the same universe, apparently. That was an adult fantasy alternate-world series that began in 1979, and was pretty progressive for its day in that in prominently featuring a romance and intimate relationship between two male leads.