What is the best filesystem for backing up on external hard drive?

I have a 1TB consumer grade NAS and I can use whatever file system I please on it. I only plan to ever use it with linux based systems, so windows compatibility is not really a priority. What is the best file system for archiving? JFS? Ext3? Ext4? btrfs?

ext4 with journaling turned on and delayed allocation (dealloc) turned off. The extents (introduced in ext4, but also available in xfs) will speed up dealing with large files--which I assume you'll be using.

Btrfs is quite stable nowadays [well, at least relatively :-) for 2.6.37 kernel] and it supports on-fly compression, amongst other nice features.

Ext4 is quite nice as well. Unless you're risky enough for Btrfs EXT4 could be good choice, but it's also not such matured comparing to EXT3.

JFS as a surprise, supports extents for a long time, formats/mounts fast and hence can be a good choice for back-ups (just remember that it has to be fsck'ed on unclean shutdown).