I agree with most said above, the Syd Barrett days, though creative I do find a bit "psychedelic" (the guy was a creative genius though totally insane !), but, Ummagumma, Atom Heart Mother and Meddle are worth a listen and also an old compilation called Replicas (See Emily Play, Arnold Lane, Bike).
The "classic" Waters/Gilmour years, definitely Wish You were Here (possibly on par with Dark Side of the Moon), The Wall (amazing concept album), Animals (Floyd Marmite !
), Final Cut (depressing).
The Gilmour years, A Momentary Lapse of Reason (passable), The Division Bell (Excellent).
There are also
many good compilation albums done over the years, most containing variations of the same tracks !
Also Keith give Roger Waters' solo stuff a listen,(much featuring Eric Clapton), particularly The Pros and Cons of Hitch Hiking, Radio K.A.O.S., and Amused to Death.
With regard to every music comment these are purely my individual preferences, but I hope it helps and I hope Dad enjoys exploring the "musical legends" !