Was always as fan of Shadow of the Empire. Fills that gap between V and VI a little bit.
Darth Plagueis is one of my current favorites and James Luceno is a great writer. It isn't cannon anymore but its the type of book that makes you say "man.. what if the Phantom Menace was more like this" which is pretty impressive to me anyway because I am more a watch it instead of read it kind of guy.
Lords of the Sith is outstanding and is also new cannon which is a plus. Paul S. Kemp is a solid writer and this story goes places that really impressed me with both Vader and Sidious. Also has a real "rebel vs imperial" vibe to it that I enjoyed with 3 stories that really come together nicely.
Advantage to these 2 are they are both "new-ish" so you might not of picked them up back in the day and likely won't be familiar with them from back in the day. Another huge plus for me is I mostly listen to books on my commute to work and back every day and both of these have GREAT audio versions if that's something that jives with you.