WORTH NOTING ON TV

THURSDAY Johnny Carson: 27th Anniversary (NBC, 9:30-11 p.m.): Even nonfans tend to concede he's the best there is at the talk-show game - breezy, jocular, tightly but unobtrusively in control - and this special marks an incredible 27 years as ``Tonight Show'' host with some top comedians doing standup routines. FRIDAY Show Boat (PBS, 9-12 p.m.): Mounted by New Jersey's Paper Mill Playhouse, this enjoyably authentic version of the classic Kern-Hammerstein musical was videotaped live from the stage, giving the program - which opens the 17th season of ``Great Performances'' - the unusual sound and feel of a filmed record.

SUNDAY The Final Days (ABC, 8-11 p.m.): President Nixon's bitter and tumultuous last days in office are the subject of a docudrama - an easily manipulated format - that is based on the book by investigative reporters Bob Woodward and Carl Bernstein.

WEDNESDAY Thomas Hart Benton (PBS, 9-10:30 p.m.): From the week his self-portrait appeared on the cover of Time magazine in 1934, he was the focus of national admiration and controversy. This Ken Burns documentary weaves together lots of material about the painter who championed the American midwest as a serious subject of art.

Please check local listings for all programs, especially on PBS.