When you say grow your bankroll, what does this mean exactly? Are you starting from $0? $10? $100? $1,000?
My answer would vary depending on the situation. Of course, from zero, you have no choice, you must start with free rolls. But as soon as you have any money in your account, you can start playing micros. Different sites off different options, but I would suggest SnGs are the best option at least at first. They're relatively low variance and a lot of weaker players play them so you should have a pretty decent edge. Once you're up to around $50 you can start playing MTT's. I wouldn't recommend playing them much before that as in and ideal BRM, you should have at least 100, if not 200 buy-ins. Once you're at $100+ I would play almost exclusively MTT's.