pick a lab where 1) you like your primary mentor (which may or may not be the PI of the lab),
2) you’ll spend a solid block of continuous time,
3) has a track record of medical students (so they know how to plug you in to the projects effectively)
and 4) has a solid chance to resulting in a paper, poster or talk based on your work while in the lab.
The last criterion is the key, because you want to show that you can be ‘productive’ and build your CV at the same time.
I’ve seen a lot of medical students rotate through labs for short periods of time and waste their time and the people training them. it takes a lot of energy to train someone in a lab, so PI’s often relegate the students to their post-docs, grad students, research scientists.
Come to the lab w/ an idea of what the projects are, and choose one specific aspect of a project to delve into. In that way, you can help free up the post-doc/research scientist/PI from that aspect and you therefore become an asset to the lab (instead of a liability).