First of all, it must be eth ofc.
Out of the polearm category, GiantThresher/Thresher is favored unless you are at 65ias in which case GPA is unarguably best. However, the differences between eth polearm bases are petty and there is no difference you can actually observe between these bases; it is just min-maxing for almost no actual difference and d2jsp fetishizing. In the end game merc dps is meaningless for most builds as well as the CB he inflicts; he is just there to carry the aura. Also don't believe arguments like "thresher has more consistent leach" etc, these are things that don't matter in real life; they exist only on paper. CV has high str requirement which means your merc won't be able to use it as early as other bases; that is something worth keeping in mind.
With 2.4 changes, I think mancatcher is the best for some scenarios like necro's who rely on merc to do the first kill and kill bosses. In this case, I'd prioritize on CB of infininity and would want a fast base. If i'm not mistaken, mancatcher is the fastest base.