Best reporting tool for RTC

From my perspective, there is no best in the world and certainly for reporting. It basically depends on what you want to do.
 

1. Jazz Reporting Service - new, emerging, reporting about multiple JTS's and applications. Easy to use. No cost, no non Jazz Tools, emerging therefore limitations.
   
2. Insight - Cognos based allows reporting across all topologies plus other legacy tools. Cost, very complex to run and maintain, complex to use, but you likely can do all you want.
3. RRDI - Like insight but runs against one JTS and the registered apps. Usage similar to Insight.
4. Birt - It comes free, but has its limitations, reports can be so badly written that they can stall your server. Since reports can not run on a separate server, this can affect your production system. 
   

Please see https://jazz.net/library/article/1490 and https://jazz.net/library/article/1489 for JRS (Jazz Reporting Service) if you were not aware of it yet.