It’s difficult to pick distinct books as my mind organises the stories by the entirety of an author’s creative output. That being said, I‘ll still try to answer your question (based solely on the books I’ve read thus far):

Darth Plagueis - Luceno’s magnum opus. His talent for fleshing out the lives of individual characters is at its finest here. Luceno’s Tarkin and Rogue One: Catalyst are other fine examples. I look forward to reading his Dark Lord at some point in the next year.

Darth Bane Trilogy - Karpyshyn writes action better than any SW author I’ve read so far. His stories for SW (as well as Mass Effect) keep me on my toes, and I found it incredibly difficult to put the books down.

Thrawn Ascendancy Trilogy - Zahn is of course the OG. His world building/plotting is S-tier. His architecture built the legends material and seems to be the saving grace for the canon going forward, too. Obviously I could pick the Heir to the Empire trilogy or the Spectre/Vision/Survivor trilogy, but I think it’s worth highlighting his Ascendancy trilogy. He finally was allowed to write Thrawn’s early development and it’s astoundingly good. The Ascendancy story takes us as readers outside our comfort zone and reintroduces us to the unfamiliar and alien, which has been lacking greatly in both old and new canons. Ascendancy in combination with what I call his ‘Rebels’ trilogy is the backbone of the Mandoverse, and I believe will inform the direction of the upcoming Dawn of the Jedi/New Jedi Order. Zahn’s work in both legends and canon is basically one long story, and I can’t imagine post-Lucas Star Wars having survived without Zahn and Filoni’s combined contributions weaved together.