A lot of discussion as to what legendaries will be good for mages, and for pure single target I can't say but it could possibly be the bloodlust one.
However I feel like a lot of them might be more fight dependant.
Bloodlust legendary for short fights.
Scorch legendary for fights with strong movement required.
And I personally think the Dragon's breath legendary will be absolutely insane for some encounters. Like Krosus for example where adds can be spread out slightly how still in a pretty slim row. Having a 40 yards dragon's breath seems to be the best thing imagineable here