I will say vmotion as this helps avoiding a downtime for hardware maintenance of the host server. There are other features like HA , svMotion , DRS which are really cool.
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I think that each version of Virtual Infrastructure/vSphere has his "best" feature.
But also depends on what do you mean with "best"
Do you mean from tecnical point of view, from user/administrator point of view, or compared with other competitor?
In VS4 I agree that FT could be the "best" feature (with all his limits of version 1.0).