Part of the plugin backlash was that it felt they were including things just to pad spec lists. They had a lot of entry level plugins from Kinetic and P5 that got dumped into Sonar. For example, of the P5 effects added, only Alias Factor really added something new.
The other issue is that Sonar has a user base which is more based towards guitar based music ("3/4 of all of our employees play guitar"), and plugins like PSYN, the Roland Groove Synth, Beatscape, and RXP don't really appeal to that crowd.
Getting back to the synths...
Rapture - clearly my favorite of the lot.
Dim Pro - I find that it has a nice palette of sounds with low memory footprint. Before I got Kontakt, it appeared on every track.
Z3ta+ - I have had more bugs with the "classic" version than I ever had with Pentagon, but the new version has been perfectly stable. I respect what it can do but I sometimes find it frustrating.
Pentagon - See my thoughts in the thread below:
http://forum.cakewalk.com/Pentagon-m3153474.aspx PSYN II - This sits in a weird space between Pentagon and Z3ta. It's not one of my favorites, but it does make some good FM bass sounds. Some of the organ sounds are surprisingly good too.
TTS-1 - I have never used this in a finished project, but, I find it invaluable because sometimes you just need a basic GM module. This one sounds a lot better than the old Roland VSC that was included on earlier versions of Sonar. All it (and Groove Synth) needs is a better menu for navigating patches.