Simply pick the fastest 5400rpm disks: Samsung F4; beats the older Velociraptor and most other HDDs in sequential I/O, due to its 666GB-per-platter density. They are 4K sector drives though, so you need to use aligned partitions. Right now this is the hottest drive, thanks in part due to its low price, a tad lower than WD EARS.
If you are going to use ZFS and have a RAID edition drive, then it comes by default with TLER enabled. You are advised to disable TLER and revert to normal recovery timeout. The reason for this is that TLER amplifies the bad sector problem and creates additional bad sectors by refusing to spend more time on recovering them. This is crucial whenever you lost your redundancy, such as when rebuilding a RAID-Z (raid5) for exampe; in this case TLER is very dangerous.
You should take care for FreeNAS to align the drives properly. Read about that on their forums and you should be fine.