Assault Rifle is one of the best for AoE, got two large cones (one fire dmg), Ignite is basically a ranged location Burn, and Full Auto does awesome damage. It only suffers due to the highly resisted form of damage.
If you wanted something a bit different, Elec has two nice AoE's (one is PBAoE however) plus a ranged nuke, Ice has two rains which you can stack together for very high damage (plus all that slow), Water Blast also has a lot of AoE with a damage patch (Whirlpool), another AoE, a cone and a ranged nuke. And then of course Fire, how could we forget that one :)