I feel like out of the smites..
Purple one is probably best, then toss up b/w red and blue. The poacher one is fairly useless for two reasons:
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It relies on a very unlikely scenario of you smite stealing a wraith/golem/buff to perfection to ever take effect.
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The XP and GOld has shifted SO heavily to small minions that I actually think it's more worth to take the small ones and leave the big one because they do so much damage but give way less XP.