Everyone here already said the pros and cons of the pets you've mentioned. To answer your following question, Nekro has the best passive damage, I believe.
SnS --> Good AoE AA, hard to control where the pools land, mainly useful in clash and elites.
Nekro --> Massive damage reduction shield, useful for PvE and PvP
MM --> Good HB (more of a warrior pet IMO), AA quite useful in elites
Pick one that suits your playstyle the best.
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