Btrfs and EXT4 were nearly tied with the Threaded I/O Tester performance. ReiserFS was in last place.
In our final test, we simply timed how long it took to compress a 2GB sample file using Gzip compression. The results were pretty close, but here Btrfs came out with the best time at 65 seconds. FAT32 was in a close second at 70 seconds while EXT3/EXT4/XFS were at 71~72 seconds and then ReiserFS at 75 seconds.
The EXT4 file-system started out with a few performance wins in this USB 2.0 flash drive testing, but by the end there were wins for some of the other file-systems as well -- even FAT32. A generalization about the best file-system for a flash drive cannot be easily made, but is largely dependent upon how the flash drive is being used and also whether data portability is needed across operating systems, etc. There are also other areas to look at with each file-system beyond just the performance numbers. Personally though I often end up using EXT4 with my USB flash drives for its general level of performance, have not encountered any issues with data loss, and "out of the box" support with most modern Linux distributions, and lack of "out of the box" support on Windows for EXT4 (a bit basic security through obscurity for those drives that are not encrypted).
For those interested in the Corsair Flash Voyager GT 32GB flash drive that was featured with this Linux file-system testing, which is waterproof, durable, offers nice performance, and is backed by a ten-year warranty, it can be found at Amazon and NewEgg for around $80 USD.