What is the best home remedy to treat a diabetic ulcer that is on the bottom of a foot?

NONE! You need to be followed by a specialist carefully to ensure that your wound does not progress and at worst become infected. Diabetic ulcerations can at times rapidly progress and result in a much worse prognosis. Follow all instructions in diabetes management and wound care as prescribed.
Diabetic foot. Many diabetics want to treat their ulcers on the feet by themselves but you should avoid doing this. This is a serious condition that requires, even if you choose to do your own remedy, observation by a clinician. The ulceration is due to underlying issues that must be addressed such as reducing pressure (called offloading). Seek assistance from a wound professional or podiatrist asap.
Wound care. . Diabetic ulcers will generally heal if you offload the area (decrease direct pressure with an insert, or a wheelchair), have your doctor trim the callus and dead tissue away on a regular basis, and if the ulcer is infected, you may need oral or IV antibiotics, depending on the severity of the infection.
Polymem. Polymem pink dressings clean and helps heal wounds. You can remove this every other day and it works well with diabetic wounds.
See a professional. Do not try to treat a diabetic foot ulcer without a professional evaluation advice. Successful treatment of diabetic foot ulcers requires the presence of adequate circulation, professional wound debridement, offloading, and infection control. Seek professional assistance.
May not be good. To treat yourself. Risk of infection and amputation. Please see podiatrist or go to wound care center.