what's the latest os for macbook pro 2.1

Well, the big question is if you are running 10.6.8 what is the compelling reason to upgrade to Mountain Lion? Is there software you need that requires Lion or Mountain Lion?

The Apple specs are clear that a MBP from mid-2007 or later is required for Mountain Lion. You're right on the borderline with a slightly earlier MBP 2,1. It was produced into spring 2007 and then replaced by the MBP 3,1 with a CPU that was only a slight upgrade, and uses the same RAM.

Beyond that, you'll just have to take the chance on whether or not the Mountain Lion installer limits installation to the MBP 3,1 or later. The place to go is the Apple App Store. Lion is no longer officially available.

According to the MacTracker database, a MacBook Pro 2,1 is limited to 10.7 Lion. There are several posts in other forums here that you can get Lion for about US$20 by calling Apple at 1-800-MY-APPLE and asking for it. They put the order on your credit card and give you a code that gets you AppStore access to Lion.

The MPB 3,1 changed more than the procesor. They upgraded to the NVIDEA Geforce 8600M GT video chipset from the ATI X1600. That seems to be the determining factor in several Mac models when it comes to runing Mtn Lion.

You may need to add RAM. Some early MBPs shipped with only 1GB RAM and Apple says the minimum for Lion is 2GB, and that's "minimum." Yours can handle up to 3GB RAM.