scook
This is an invitation to a gunfight. FWIW, Z3TA+2 has never been included with SONAR. It has been included in special bundles, like the X1 Production Suite. The original version of Z3TA+ is bundled with SONAR. I expect each synth has it's champions and the answer may depend on the musical style. My long standing favorite of the Cakewalk synths is Rapture.
Gunfight? No way. This will be much bloodier, lol. I not really sure if the OP meant the best Cakewalk synth ever in Sonar or the best synth period ever included in any version of it. You were quite right to point out that Z3TA+2 might not belong in the conversation, but it seems to me it should be.
Rapture is a great sounding synth, Dim Pro I still use but I hate the gui's of those and Z3TA though they all are very underrated.
But the mere remembrance of Dream Station set my teeth on edge. I hated it. Sonar 1 XL was my first experience with soft-synths and I kind of needed something like the DS but it insulted my ears and I thank all deities I found great free synths that sounded sooo much better, but that also led me to the Triangle1 & 2 and The Pentagon which was the first synth I ever bought.
This is so subjective, but if I didn't have anything else other than CW synths and I had to choose one I'd have to go with DimPro because of the content, but I'd kick myself for not choosing Rapture.
But if we're talking about the best synth ever included in Sonar (back to 1XL) it's hands down Tassman3 and it's not even close.