Depends on how much you want to alter your driving habits, lol.
1-1.5" is a good drop for a daily driven car...you lose the 4x4 look but can still enter driveways. Eibach pro kit and Ford Racing springs are a good choice for that drop.
1.5-2" is the more extreme drop...you will most likely want new shocks/struts, as well as caster camber plates, ball joints and a bumpsteer kit. I recently lowered mine ~2" in the front and 1.7" in the rear with H&R Super Sports. I love the drop and the car handles much better, but I've really had to alter my driving habits...no more driveways with steep inclines and I usually stay away from low curbs and parking barriers.
12 Black GT500 SVT PP l 2.6 Pulley/Idler l Airaid l o/r X l 305 NT555R's l Recaros l
04 Azure Blue Mach 1 / 03 Sonic Blue 3.8 / 14 Sterling Grey F150 FX4 402A 5.0 / 11 GT500 R.I.P.