WHich GitHub plan is best ?

I am deciding between personal and business paid plans. I have one developer on my team.  I might get one more. WHich options is best for my startup?

As you say you have currently only one developer on your team, So for startup personal plan is good for your firm.

Personal Account Provides:

Unlimited public repositories
Unlimited private repositories
Unlimited collaborators

In future, if more developers join your organization then you can switch over with Business Premium plan,

The business account for GitHub enterprise also provide 45 days free trial, so if you have still any confusion you can use a trial for 45 days and then take a final decision.

For Pricing plans visit:

https://github.com/pricing