Dear Therin of Andor: I like alien contact (especially the Borg), hard tech. I like almost all the characters. My favorite episodes of TNG is The best of both worlds, Q, who?, First Contact; almost all, except for a few. TOS: Where no man has gone before, Space seed, The city on the edge of forever. VOY: I can't remember, I haven't watched it for some time; maybe when Seven of Nine is caught and transformed back to human. I prefer to buy new, but if not available, used is OK too. Thanks in advance.
Okay, that makes it easy then. Without any hesitation, I recommend "Vendetta" by Peter David, a sequel to TV's "The Best of Both Worlds" (TNG) and "The Doomsday Machine" (TOS). It had a recent sequel featuring VOY's Admiral Janeway and Seven of Nine, but that was met with mixed reviews: "Before Dishonor".
If you want more Kirk & Gary ("Where No Man...", TOS) stories, then look for the trilogy "My Brother's Keeper" by Michael Jan Friedman - it's great!
For Khan stories, there's a great duology ("The Eugenics Wars: The Rise and Fall of Khan Noonien Singh") and a sequel, "To Reign in Hell") by Greg Cox.
As for "City of the Edge..." (TOS), "Crucible: McCoy", by David R George III, tells what happened to the McCoy who saved Edith Keeler's life and was forever trapped in the past. It's a huge novel, but well worth getting lost within for a week or so. The people I stayed with while reading it thought I was being very unsociable.