"Best" is very much a matter of opinion.
The first decision you should make is what sort of timer do you find easier to read. Bars or Numbers.
That will then narrow down the choice somewhat, which still really depends on too many factors to be a simple decision.
WeakAuras, PowerAuras, TellMeWhen, NeedToKnow, ForteXorcist, Raven, to name only a few of the common ones which spring to mind.