What is a Death Knight?
A Death Knight is a plate wearing class that can tank and DPS with any of its 3 trees with varying degrees of success depending on the situation. Each tree has its own specialization and style of tanking. There is no "best" tree (although some would argue otherwise) - you just need to pick the one you like the most.
The current focus of Blood is physical damage combined with the ability to self heal through talents. This is mostly staying the same in 3.1. In its current incarnation Blood has great single target threat, but rather lackluster AoE threat. This will change with 3.1 and the removal of Lichborne as a tanking cooldown - together with the changes to Unholy will greatly improve the AoE threat of Blood. On top of that, Blood receives a huge single target increase from the changes to Sudden Doom, and the bonus disease damage changes to Heart Strike and Obliterate.
Frost is focussed on mitigation and Frost damage (duh!). It has the best overall physical mitigation of the 3 trees, together with great single and AoE threat. In 3.1 this largely stays the same. Some talents are getting changed, but the overall focus and threat abilities are staying the same.
Unholy has the best magic mitigation of the three, but the poorest ability to deal with physical damage. On Live right now, Unholy also suffers from horrible single target threat compared to the other trees, but extreme AoE threat from Unholy Blight, reduced CD on Death and Decay, and long disease duration. All of this is largely staying the same in 3.1 - however Scourge Strike is being buffed to take care of the single target threat problem.
All in all, you have to try each of the 3 trees and decide which one you like the most.