Despite the man's real life politics, I really enjoyed the first Ender's Game, and the rest of the series is pretty good even if it goes in a totally different direction. Never did get round to the Shadow series.
Forever War is brilliant, the sequel Forever Free does a similar thing. The thematic sequel Forever Peace explores the same ideas but in a slightly different universe. There's apparently a short story that explores the space between War and Free from a different character, but I literally just found out about it while I was looking up the names for the sequels.
I've always meant to try the Honorverse by David Weber, but have never got round to it.