There are DOZENS of threads discussing "must-read" plays. Do a search and you'll discover a great compilation of lists.
If you're a stronger actor in a specific genre, begin there. If you're a great classical actor, read the Shakespeare canon, all the great classical playwright's works, and add Peer Gynt to that list.
If you're stronger in contemporary works, go for LaBute, Letts, Ruhl, Vogel, etc. If you're female you will definitely want to ready the works of Ruhl, Vogel, and especially Wasserstein.
And if you're stronger in-between, Williams, Miller, etc.
Some of my favorite new and old plays:
THE CLEAN HOUSE by Sarah Ruhl
AUGUST: OSAGE COUNTY by Tracy Letts
THE VISIT by Durrenmatt
THE MAN WHO CAME TO DINNER by Kaufman & Hart
DOG SEES GOD: CONFESSIONS OF A TEENAGE BLOCKHEAD by Bert V. Royal
THE PILLOWMAN by Martin McDonagh
THE SEAFARER by Conor McPherson
BLITHE SPIRIT by Noel Coward
THE ZOO STORY by Edward Albee
"The Spectacle has, indeed, an emotional attraction of its own, but, of all the parts, it is the least artistic, and connected least with the art of poetry. For the power of Tragedy, we may be sure, is felt even apart from representation and actors. Besides, the production of spectacular effects depends more on the art of the stage machinist than on that of the poet."
--Aristotle