how can I choose the best bounty
Before you choose a bounty you just have to look at the white paper. When it terms to the white paper it is the most trusted reference everyone can rely on. Also, you can rely on other sites but be careful if it is useful or scam.
Make your research on the best bounties you think is good and check for the team, whitepaper and other relevant information that such bounty is legitimate. Just sharing my way of choosing my bounty campaign, try checking for those higher rank member's avatar on this forum and list it down then you can start checking it one by one and gather the information that you need. Check out the spreadsheet if there are many high ranking members joining to such bounty and check also when it started until it will end.