Tbred 2600+ if you won't overclock, as it's a bit faster than a 2500+ at stock speeds.
Overclocking, I would recommend a 2500+, it's a little cheaper, and overclocks to 2.2 - 2.3GHz aren't uncommon.
That said, the 2600+ isn't a bad overclocker itself, quite a few make 2.3 - 2.4GHz if you have...