Cardassians- as discussed at length in my Cardassian philosophy thread.
Romulans- the modern novel's Romulans, which are a mix of TV Romulans and Rihannsu, come across very complex as as result of these different influences. Sherman-Shwartz and Mangels-Martin are top of the field here.
Andorians- The post-finale DS9 series made them quite fascinating, and now they've been reconciled with Enterprise
Andorians, they've achieved similar status to the Romulans (above). Heather Jarman is top of the list here.
Bajorans- "Day of the Vipers" was the best portrayal of the Bajorans I've yet seen. James Swallow captured them perfectly.
Klingons, when written by KRAD, who knows how to make them complex, fascinating and three dimensional.
As for beta-status aliens, I have great fondness and interest in Efrosians, Pahkwa-thanh, Nasats, Tellarites, and pretty much any aliens created by or explored by Christopher Bennett.