As a sequel to my earlier post, I will add a few I forgot last time:
The Tholians, as featured in "The Sundered" and "Vanguard" have been very interesting
I'm quite partial to the Betazoids, but we haven't yet had the
Betazoid book ("The Battle of Betazed" and "Imzadi" (one of the relatively few pre-21st century Trek books I've read) were both good, but didn't really elevate Betazoids to the height I'd like to see.
The Trill were made very interesting once writers began focusing on their secret-keeping/conspiracy traits, particularly playing it against their promotion of memory and knowledge.