New Earth: This was one of my first mini-series, and I have good memories of it. Book 1 and 2 serve as a good introduction to the series, and Wagon Train especially has remained in my top 10 ever favorites list since I first read it. Book 3-5 are somewhat repetitive in their continued rehashing of threats against Belle Terre, but at least nr 3 was fresh in its use of the minor TOS crew and planet-based events as the driving force. Book 6, Challenger, was my first book using primarily original characters, and like Wagon Train, I have very fond memories of it.
Double Helix: I've read about half of these. Vectors, the Pulaski story, was a quick but very entertaining read that also gives some glimpses into early DS9. Double or Nothing, the Calhoun story, is the only other I can recall. It's nice if not one of New Frontier's best, and can be read even if you're unfamiliar with the NFR series.
Captain's Table: Once Burned was very good, and as my first NFR book, I can recommend it as a starting point to the series. The only other one I recall is the Sisko story, which was a real struggle to get through. The later anthology has a few nice stories as well, those vary a lot so there´s bound to be some to please everyone.
Mere Anarchy, Crucible: Not read these.
DS9 Millennium: Decidedly different kind of narative. Epic story setting, but takes some getting used to. My only real problem with it is how Kira is written, which doesn't mesh well with the part of DS9 in which the series is set.
Gateways: (not read TOS and VGR entries). The TNG, DS9 and NFR entries are somewhat connected, which I liked a lot. The TNG entry is a light and quick, but entertaining read. The DS9 entry remains one of the highlights of the early relaunch novels for me, and is a must in the relaunch continuity. The NFR entry was met with mixed reviews here, but for me was nice enough - it's classic NFR style and I enjoyed what M´Ress and Arex brought to it. The Challenger entry is quite a unique story, I'd say must read if and only if, you've read New Earth first.
Genesis Wave: At the time a nice read, but I'd say the quality of later novels has not been kind to Genesis Wave. The size of the crisis in these really falls flat a bit given its lack of long-lasting consequences.