The Summoner
Never ones to travel without an entourage, Summoners have mastered the ability to control Demons as well as the very elements of nature. These forces can then be unleashed to distract and destroy their enemies, as well as provide protection for the caster and his allies.

Summoners rely heavily on the minions that they conjure, so consider the advantages and disadvantages of each before deciding which build to go with (the Warper seems to be a popular choice). As for weaponry, you'll most likely find yourself seeking out a couple of skill-enhancing legendary weapons or a nice unique like The Master's Hand.
Summoner Skill Advancement

Level 1: Afterlife (A1, B1), Summon Carnagor (F1, G1), Summon Fire Elemental (C1)

Level 5: Blood Link (E1), Enrage (G2), Master of the Flame (C2), Meat Shield (F2), Word of Fear

Level 10: Blood Surge (A2), Drain Life (B2), Summon Spectral Elemental, Summon Witch Doctor (H1, I1)

Level 15: Dominate (I2), Hand of Nostrum (H2), Summon Force Elemental, Summoning Circle (E2)

Level 20: Brom's Curse (B3), Elemental Nova (D1), Summon Storm Elemental, Summon Warper (J1, K1)

Level 25: Blink (A3), Spectral Strike (K2), Spellstorm (J2), Summon Toxic Elemental

Level 30: Elemental Drain (B4), Master of the Elements (D2), Summon Reaper