Earth is going to be the best from a control standpoint, simply because it has multiple strong AoE controls.
 
	 
 
	Plant is a close second in most content.  Seeds of Confusion is way overpowered against most enemies (note: Nemesis have inherent confuse protection). However, other than seeds your AoE control is a little lacking.  What makes up for this is Carrion Creepers, it's kind of control-y but also does decent damage.
 
	 
 
	I'm not going to comment on Gravity, as it just doesn't jive with me.  I like the looks of it, but I never really get into Gravity controllers.