If you could rate ticklish spots from 1-5, what would you rate them on average? Here's a scaling system:
? - you don't know if you're ticklish there.
0 - not ticklish at all, deadpan reaction.
1 - barely ticklish, just enough to twitch around a little and a smile.
2 - slightly ticklish, light giggling.
3 - moderately ticklish, heavy giggling to light laughter.
4 - very ticklish, moderate to heavy laughter.
5 - extremely ticklish, hysterical laughter.
Here are mine:
Behind knees: 
Feel free to add other areas, but these are the core, traditional ones. I know I've opened myself bigtime here, hehe, but I hope my bravery has set an example!