I haven't tried Lily, but out of the other 3 I would probably go with Siri. I don't find any of them difficult to maneuver and use though. With Siri I can simply press it against my body with my palm. The shape is designed well to stimulate my clit and labia when held that way. With MiMi and Form 4, I hold the toy in my hand and wrap my fingers around it, pressing the tip into my clit. That isn't a problem for me at all, but Siri seems more ergonomic.

If you're looking for something to use during sex, I find Form 4 can get in the way sometimes since it's larger than the others.

I would say Form 4. The buttons are raised and easy to find, but don't get hit accidentally. I don't find MiMi's buttons too hard to press, but I don't like how I have to look to check which is + and -. With Siri it's easy to tell which button is which, but I find they are a bit too easy to press and sometimes I hit them accidentally.

We just bought a Mimi and like it so far. We have not tried the others you have listed, but can say that the Mimi is better than the Bcurious, in our opinion.