There are many good options nowadays, but it would really depend on what you are looking for in an online book.
Here's what I would consider:
#1) I would say security and reliability. Would be good to check which one has many years of operating in the market, which is usually a good indicator.
#2) Bonus and welcome offers. This could really make a difference if not sure for which one to go, make sure to check on all bonuses available at the time, as some are better than others.
#3) Banking. Very important to review first which book has the banking service you will need for your particular bank account, credit card, or currency you would like to use (even Crypto!)
#4) Check on several different online reviews. You can find this to be a little overwhelming as there are tons of reviews. Not sure if I can post a link here (if not please have it removed), but I can recommend the Best Sportsbooks Rating Guide that is available at the BMR site.
https://www.bookmakersreview.com/best-sportsbooks/ I've found this list to be very useful. Hope it helps!