My personal favorites with brief descriptions (only for the first few now, but hopefully more to follow):
That Obscure Object of Desire (1977) - Bunuel's final and my personal favorite of his films. While basically surrealist, with the added wrinkle of two actresses playing the same character, this is one of Bunuel's more conventional narratives outside of the soapier Mexican "studio" films he helmed, and certainly one I'd recommend to both the budding Bunuelophile and someone who'd never seen any of his films.
El (1953) - an odd choice for this high up the list, perhaps, but a fascinating narrative that contains most of the great Bunuel tropes: the anti-clericism, the sexual fetishes, the fragile male ego and the jealousy/paranoia stemming from sexual inadequacy. The "needle and thread" scene is a wonder -- despite being merely suggestive of his intentions, I've never known anyone who didn't get exactly what he was up to. Not as easy an accomplishment as one might think, given the obvious constraints. Clearly a big influence on Hitchcock with a clock tower scene that brings to mind "Vertigo", it's a great film by any standard and likely to please the casual viewer.
Viridiana (1961) - appears at no. 3 here, but this may be my favorite Bunuel film. I say "may be" as I've only seen it once, back in college when I was just getting into Bunuel. But it's stayed with me ever since as something of a cinematic touchstone, a film that truly opened my eyes. It's been 17 years now since I've seen it (and I'm tempted to order that French disc with no English subs...).
The Milky Way (1969) - the ultimate anti-clerical film by Bunuel, but may appeal more to philosophy/religion students than the more casual viewer.
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The rest of my favorites, more or less in order. I really dig all these films a lot, with the single exception of "Robinson Crusoe" (1954), so aside from that one consider them basically all bunched up in a big ball of recommended goodness (that is, I don't love "The Exterminating Angel" ten times more than "Las Hurdes"):
The Exterminating Angel (1962)
Un chien andalou (1929)
The Phantom of Liberty (1974)
The Discreet Charm of the Bourgeoisie (1972)
Los Olvidados (1950)
Belle de jour (1967)
Diary of a Chambermaid (1964)
Rehearsal for a Crime (aka The Criminal Life of Archibaldo de la Cruz) (1955)
L’ Âge d'or (1930)
Tristana (1970)
Las Hurdes/Land Without Bread (1933)
Robinson Crusoe (1954)
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I have not seen any of the following (and I'm dying to see "Nazarín", in particular!):
Nazarín (1959)
Simon of the Desert (1965)
The Young One (1960)
Fever Rises in El Pao (1959)
Death in the Garden (1956)
The River and Death (1955)
That Is the Dawn (1955)
Wuthering Heights (1954)
Illusion Travels by Streetcar (1954)
The Brute (1953)
A Woman Without Love (1952)
Ascent to Heaven (1952)
Daughter of Deceit (1951)
Susana (1951)
The Great Madcap (1949)
Gran Casino (1947)