While I've never actually "seen" a ghost. I've had multiple ghostly encounters. I'll mention the 2 creepiest ones.
First, when I was 17, my friends and I heard about an old abandoned farm house that was supposedly haunted. So we decided to go check it out. The main living area was fine. No sounds, no creepy vibes, just a dark empty house. The basement was a different story. It was small; maybe 12x15 feet. No windows. Nothing was down there except a small table. We were down there for maybe a minute, when all of sudden one of my friends started screaming. He rips his shirt off, and there's 3 large gashes in his back. We quickly looked around to see if maybe there was an animal or something that could've attacked him, but there was nothing. We quickly booked it out the house.
Then, a few years later, I was living in a country home that belonged to my grandpa. He was on vacation in Florida, so it was just me living there. The house was secluded. Closest neighbor was about a mile and a half down the road. One evening, while in the shower, I heard a knock at the bathroom door. I was startled, but figured it was my mind playing tricks. But it happened again a minute later. There was no mistaking this sound. It was clearly the sound of knocking on that door. I jumped out and opened the door, and there was nothing. I searched the whole house. There's no explanation.