The Rubicon Express 4.5" long-arm suspension I installed into my previous TJ. (long arms kept hanging the TJ up on the rocks, the geometry was bad which caused my front-end to unload and want to lift up too far on steep climbs or tough obstacles)
Edit: I'll add a few more notables...
Teraflex control arms (not flexible enough, tore two control arm mounting brackets off)
4.10 gearing with 33" tires and my previous TJ's 5-speed transmission (left the Jeep undergeared)
K&N Air Filter (passes way too much desert dust & dirt)
All Terrain Tires (not aggressive enough for my wheeling areas)
ProComp ES-3000 shocks (way too stiff)
Nerf bars (get in the way, offer no protection against anything)
But never an inclinometer (shows useless misleading information)