Probably the one I re-read the most with fond memories is Hellspark by Janet Kagan.

A series I enjoy because I felt it captured the sheer horror of the xeno-menace the best was called The Vang series by Christopher Rowley, in particular book 2 always gives me the heebeejeebee's.

For more modern books, the Cradle series by Will Wight that just wrapped up has been a fun time for me and I re-read my favorite sections often.