Anything you use frequently, or daily. Saddle, bridle, saddle/half pads, boots/polos, etc. I also suggest keeping your grooming tote, tack cleaner, and your horse’s blankets.
Cooler blanket, quarter sheet, saddle pads, brush bag, Mane and tail shampoo + conditioner, fly spray, saddle, saddle cover, lunge line, crops, drawers for medical stuff, treats, water bucket, stepping stool, bell boots, tendon boots, leads, girth, extra halter, gloves, Spurs if you use them, and your helmet.
I would recommend your everyday saddle pads, saddle(s), bridle(s), breastplate, tack cleaner, grooming kit, helmet, boots/polos, crops/whips, treats, and a horse first aid kit.