The flats I've tried myself...
Rotosound 77's have pretty much the highest tension out there for flats. They also are very bright and punchy compared to other flats, and have a rougher feel, so they aren't everyone's cup of tea. I am currently using them on my Ibanez SR505.
D'Addario Chromes are slightly lower tension than Rotos, and slightly less bright, but are still brighter and tighter than most flats. They have a good "general purpose" sound for most kinds of music. Lots of people here like Chromes.
Fender 9050M's have a medium tension, less than the Chromes but more than DR's or TI's. They add a warm depth that underscores Fender bass tones very well; I use them on my Fender Aerodyne J and highly recommend them for my fellow Fender fanatics.
Hope that helps.