Sonar comes with a total of 15 softsynths. My favourite are DimensionPro, Rapture and Z3ta.
Z3ta is a very powerful softsynth. You can upgrade it to Z3ta 2(nicer GUI and 1000 more preset sounds). If you are looking for an inexpensive but awesome softsynth you can also look at Synthmaster. (ranked number 3 by MusicRadar).
There are also a bunch of decent free ones on the internet. My favourite is the P8. Many people like the synth1.
Also check out
http://kunz.corrupt.ch/. This is actually Tagu Audio Line. They are famous for making some free analogue vst synth. Their Juno 106 emulation synth is the TAL-U-NO-LX. You can download their free demo.
Hope that helps.