NTFS. It's been the most reliable FS I've used (though EXT4 has been solid, so far!), and HFS+ sucks in Windows far more than NTFS does in OS X.
NTFS may be slower than HFS+, but usually, performance is lost at a higher level or lower level than the FS, except in heavy server loads. FI, turning indexing off in Win7 with a platter drive will nearly get rid of pauses when multitasking, or trying to do work while copying a bunch of files.
As of Windows 7 and mature 2.6 kernels (including 2.6.40 and up, aka 3.x

), FS choice really only matters for the reasons it aught to: how safe your data will be, when commodity hardware and software fail on you (an area where EXT3 sucked monkey balls, despite great reliability in your typical RHEL clone server with RAID, backup, and a UPS), an area where NTFS has excellent since back in the NT4 days.