A few options, as I see it:
1. Use jflow (requires an external collector, or does it?*)
2. Use "show security flow session" and look for the highest byte/packet count (not really easy to use with a decent number of flows)
3. Use "port mirroring" or analyzer session and send data to a sniffer for top talker analysis.
Any other ways?
* Is there any way to view jflow data locally without using an external collector? Similar to Cisco's "show ip cache flow" for netflow? I see 'set forwarding-options sampling family inet output file' option, but that never gets populated with data.