This is really a two-part tip. I try to add as little sugar as possible to my banana bread. This means packing as many naturally sweet and creamy bananas into my bread as I can. So, when a recipe calls for 2 medium bananas, I will usually use 3. The more the merrier, in my opinion, and I swear that this tactic has never steered me wrong. The bread bakes up just fine, stays super-moist and has intense banana flavor.
When it comes to choosing sugar, I always go for light or dark brown sugar: brown sugar adds a deeper, more caramel-like flavor to my bread than granulated sugar, which has a less nuanced flavor.