"Depends.... Radio telescopes have the record now... Keck has the biggest mirror and highest resolution (I think) in the optical... There is the 100 meter OWL planned but unbuilt... And the FHT telescope is next...(joke there...guess what FHT stands for)"
(hmm, Fantasically Huge Telescope?) OWL is: OverWhelmingly Large.
Keck presently has largest "single" telescope (though it's built of multiple segments. Though Cerro Paranal with it's 4 scopes using Interferometery are actually the largest opitical telescope and has highest resolution- but all 4 haven't come on line yet.
"Bigeyes" is listing of all major optical telescopes made and in construction:
http://www.seds.org/billa/bigeyes.html