I think what makes a great business partner can vary from business to business and founder to founder. I think the key thing is that you need to have someone who you generally get along with well on a day to day basis. You need someone who you can trust, and who you know will dedicate themselves to building the business. Someone with complementary skills is also a big bonus. I know that in our company, it always seems that problems I am not capable of solving, my co-founder is capable of solving, and vice versa. This better helps our company tackle complex challenges and get work done efficiently.
It's also important to have a co-founder who is going to challenge you. Who will constructively argue with you over important issues and make sure that you are thinking about things critically.
One piece of advice I would give is that I would caution someone who is searching for a co-founder, with whom they don't already have experience working with. I believe that it's comparable to being on a dating site and looking for someone who you plan on marrying right away. I think that it is extremely beneficial to have had prior professional experience in working with your business partner, as it's important to know exactly what their skills are, how dedicated they are at work, and whether or not you'll get along over the long term.