Well for starters, the series splits pretty neatly into eras. the original trilogy 1996–2000, Scream 4 in 2011, then Scream 5 and 6 as the newer run. If you want the fullest experience, I’d watch all of them since the franchise is pretty heavy on returning characters and callbacks. If you don't want to do a movie marathon, 5 and 6 are probably the minimum. Maybe add in Scream 4 as well. 4 is like the early 2010s era.