Tristana (which is what I meant to say above, not Viridiana -- which is an earlier Mexican-period film) is quite a straightforward story, with surrealistic overtones but basically a conventional film about a young woman and her elderly guardian's intentions for her.

That Obscure Object... is very funny, but made slightly confusing by the lead female role being played by two different actors, for no very apparent reason. If you can get to grips with that, then it's enjoyable. It's told mostly in flashbacks but is a simple story of falling in and out of love.

Discreet Charm... is also funny but rather silly. A group of friends are trying to have dinner together, but they keep getting interrupted by more and more bizarre occurrences. This is quite Monty-Python-esque I guess.

Any of those would do to start, whichever sounds more appealing! Although I think once you've seen a few, and have more of a feeling for his world-view, then they all start to make a lot more sense. (I was lucky in that when he died, BBC2 showed all the major films one per night over a fortnight or so, and I was the right sort of age to be able to stay up and get thoroughly immersed.)