If some skin (that might not be what it is called) on the glans at the end of someone's penis got ripped off, would it grow back? If so, would it grow back nicely, or would there be a scar there?
It will grow back, but whether there will be a scar depends on the size and depth of the tear. If such happens, the wound should be covered with antibiotic ointment several times a day until it heals.
What kind of antibiotic ointment?
The kind doesn't make much difference. The big concern is keeping the wound from getting infected. The genital region is generally warm, moist and too good of a breeding ground for germs, so the ointment will seal off the wound and help promote healing.
What would be the worst that could happen if it got infected? And what would most likely to happen if it got infected?
Do you think it would be okay to leave it without putting ointment on it? if so, how long do you think it should take to heal? Also, how long do you think it should take to heal if there was ointment on it?
Preventing an infection is easier than dealing with one. "Worse" is open-ended in this case. It would depend on what bacteria got into it. If it gets infected, the area will become red, sore, likely to ooze, and might even feel extra warm to the touch.
How long it will take to heal is a question I can't answer because I don't know the nature of the wound, nor how big or deep it is. Ointment tends to increase the healing speed and it will tend to minimize scarring.