Okay. So I'm biased because I'm the author of Flask, but here something to help you make the pick:
Besides the ones you mentioned there is also Bottle which is similar to Flask but more minimalistic. Unlike Flask it also re-implements everything from ground up instead of building on an independent foundation like Werkzeug.
Other alternatives is web.py, one of the first microframeworks ever. Same rule as bottle: re-implements everything from the ground up.