A LOT of commercial television features programming with a science fictional content; think of "Amazing Stories" and "Twilight Zone" or such popular animated serials as the Transformers and Voltron. But for the true sf fan there is only one really worthwhile series -- other than "Star Trek" reruns -- and it comes from public television: "Doctor Who." Despite appearances -- slouch hat, 10-foot-long scarf, and pocket full of jelly babies -- he is the greatest of all the Time Lords.

Along with a lissome female companion, each Sunday the Doctor thwarts some fiendish plan for world domination, conquest of the galaxy, or even destruction of the universe. Traditional villains include the Daleks -- robots programmed to destroy organic life -- who go around chanting "Exterminate, Exterminate"; the Master, a renegade Time Lord of immense power; and a variety of aliens, evil scientists, bloodthirsty cults and creatures from other times, places and dimensions.

All these are pretty familiar figures of sf lore, but what makes "Doctor Who" special is that they are treated in a lovingly ironic fashion. As played by Tom Baker, the Doctor displays a Candide-like naivet,e, combined with wry humor, great gusto and the lovableness of a teddy bear. The program's fans are legion and rabid, and with good reason: "Doctor Who" is generally regarded as the finest sf television series ever.

DOCTOR WHO -- ordinarily appears on Sunday at 1 p.m. on Channel 22 and at 5 p.m. on Channel 26. The strung-together segments normally run 11/2 hours.

STAR TREK -- generally runs Saturday at 10 a.m. on Channel 5 and Monday through Friday at 6 p.m. on Baltimore's Channel 45. Times for both it and "Dr. Who" do vary sometimes, however, so check your TV listings before popping the popcorn.