Yeah, there's a bit of an echo in here, but the big ones to start with if you've read none of them are definitely the original Thrawn Trilogy (Heir to the Empire, Dark Force Rising, The Last Command) and X-Wing series. They're the two biggest standouts of the EU, and take place not to long after Return of the Jedi, so they're the perfect starting point too.

After those, the Hand of Thrawn duology (Spectre of the Past, Visions of the Future) is solid, and covers the actual conclusion of the Galactic Civil War (meaning the war between the Rebel Alliance/New Republic and Empire/Imperial Remnant). Anything by that author, Timothy Zahn, is generally solid, with other novels he wrote being Survivor's Quest, Outbound Flight, Allegiance, Choices of One, and more recently the new-canon Thrawn series (Thrawn, Thrawn: Alliances, and Thrawn: Treason - though I haven't yet read that third one, just got it as a gift recently) that puts that character as its protagonist. Bonus points, the first of that new series fits just easily into the old EU as it does into the new canon.

Honestly, it's been so long since I read most of the other EU novels that I can't recall enough to give many thoughts or recommendations about them. I do recall liking I, Jedi, which stars Corran Horn, one of the main characters of the X-Wing books, and is probably the only Star Wars novel written in the first-person. But even that I couldn't much say why I recall liking it beyond that I liked Corran. I also recall enjoying the Junior Jedi Knights series, which were children's books about Anakin Solo's training as a Jedi at Luke's Jedi Academy, but I did read them back when I was still in their target demographic age range, which is quite young IIRC.

Beyond that, well, the books are actually fresh in my mind are the more recent ones, from after the New Jedi Order series, which were... not great. Only recommend those if you want to read everything.