What is the best laser for tightening up a saggy butt? Venus Legacy, Venus Freeze, Coolsculpt or Titan?

The best butt tightening will come from a Brazilian Butt Lift (fat transfer) or a surgical excision of loose skin.  For cellulite, consider treatment with Vaser Smooth.

Without photos and knowledge of your age it is difficult to tell. All treatments you mentioned are non invasive . Some reduce fat while others are supposed to tighten skin . Skin tightening with non invasive treatments have limitations on the degree or amount of skin that can actually be tightened . You may need volume more than anything , which some of these treatments by reducing fat will Make your situation worse. The gold standard for skin tighten is usually a surgical lift . It is best  to seek a formal consult from a plastic surgeon and see where you fall in the spectrum of skin laxity.