You know how it is. If you mention a writer you like to someone without suggesting a specific work, your friend will invariably be drawn to the worst hackwork that writer has ever perpetrated—some Scooby-Doo tie-in book, or a late completely unedited doorstop bestseller dedicated to a much younger third spouse, or the subsequent comic novel set in academia with a recognizable grudge character based on that unfortunate late-period spouse.
But recommending the best, or most representative work might not be a great idea either. Any more than the first book of a series is always the best choice. Some writers get worse as the series goes on. Others get better. Some hit slumps and then come back stronger than before.
For example, I discovered the mystery writer Reginald Hill’s Dalziel/Pascoe novels midway through, with Bones and Silence. That turned out to be an excellent choice. That middle period of D/P novels, from that book to On Beulah Height (including Recalled to Life, Pictures of Perfection, and The Wood Beyond) are a perfect blend of detective novel and literary game playing. Before that they are more standard, though still good; past that the literary game playing takes over and they make less and less sense as actual detective novels.
Sheer luck on my part. Otherwise I might not have taken to him like I did.
When Robertson Davies’ The Rebel Angels came out, a reviewer said it was fine, but that Fifth Business was much better, and the best place to start. The reviewer was right, and I’ve since read almost everything Davies has ever written. Even the less-good stuff is great, but I might not feel that way if I’d tried to start with The Rebel Angels.
One collection of recommendations for mystery novels I saw posted in a mystery bookstore recommended starting the Lord Peter Whimsey books with Gaudy Night, possibly the best way to put someone off Sayers for life. I started with Strong Poison, perhaps not the best place, but it worked for me. I’d say start before Harriet Vane appears, probably Murder Must Advertise, and work your way toward her. Not Five Red Herrings either, unless you have a fetish for train schedules.
Pynchon? The Crying of Lot 49 is an obvious choice (short!), but I’d say V.: if you can’t take the length, stay out of the Pynchon.
David Foster Wallace? The Broom of the System (TCOL49 updated), his two great early travel essays “A Supposedly Fun Thing I’ll Never Do Again” and “Getting Away from Already Being Pretty Much Away from It All”. Try a story, and if you don’t like it in a few pages, try another, because it’s not going to change. Wallace short stories are weirdly isotropic, no matter how long they are. They’re the Red Queen’s Read: you’ll run through a lot of words but you’ll end up in the same place. If you like that place, great. If you don't, leave.
And Jack Vance, mentioned in several other posts? I'd say The Star King, then into the Demon Princes series, though The Blue World is a good standalone. And here's where reader preferences really do need to be consulted. Someone with a bent to fantasy might prefer Lyonesse. But the wrong choice can really put someone off him. That's actually what started me thinking about this. Years ago I recommended Vance, and a friend went to The Gray Prince, a weird and twisted little work, a precursor to his later "cosy genocide" manner, which I found a bit off-putting, for Vance completists only.
So, if you can, ask. And heed warnings as well as recommendations.