What was your first invertebrate?

I honestly added in snails first since I figured they were the hardest to kill while I was still testing out doing water changes, etc. from mixing the salt into the water at the time (I know, painful, but my local stores didn't have any of it pre-mixed sadly).

hermit crabs were mine since I figured that snails would be easy but may become a pest unlike hermit crabs

Probabky the easier, I tried snails out to make sure my levels were safe enough for other and for fish before I stocked my tank.

Snails. As others here have said, I wanted to start out with what I thought would be easiest.