For me, it was a friend of my elder sister who used to tickle me when I was younger (say till I was 12-13). She was 15-16, same school so she would visit our home often to meet my sister and at times would overpower me (obviously) and dish out insane ticklings. I would often stay away from her...
Another very ticklish feeling I feel is when at the salon, as part of the massage, the hairdresser uses a scalp tingler (maybe the only thing that can tickle my head, especially when the beads go behind the back of the head towards the neck.
Hey! I share the thing about shirt and grass too! The grass does tickle on the centers of feet and when it goes between the toes. And about a shirt (or any kind of loose top), when there is a breeze which causes it to flutter against my belly, it certainly gives a very ticklish sensation!