Hi,
It really depends on what you like and technologies you know.
In example if you like NFV or SDN, may want to find a related project as Tacker, neutron drivers, etc.
If you are more from deployment or operation, may want to contribute to deployment projects as Kolla, OSA or infrastructure services as Zuul.
In the other hands depends on your develop knowledge, if in example you are experience with java, Monasca is one project written on it. Most of the others are written in python, and deployment projects with ansible, puppet, chef, etc.
As resume, if I was looking for a project to contribute, I'd start thinking what are my preferences and my knowledge, if you choose a project related to a technology you don't like, may stress you and end up leaving the project.
- Pro-tip: Smaller projects are easy to contribute and receive feed back from the team, bigger as Neutron and Nova are hardest to get involved.
Hope it help, Regards