K, I will be the first one to say it. You have time, do some research. There is no "BEST" spec for a DK tank. There are plenty of thread on this site and mulitple other sites that will help you out.
Take the time to do some research and a good amount of reading. You will soo realize that the "BEST" spec, will be the one that fits your play style. Your only lvl 63, you have plenty of time to do the research.