The Hero Hunters Best Team is going to be made up of the best heroes that you can get your hands on. You going to want at least one from the different attack types so you can have more of an advantage over enemies. The more stars that a hero has the more power they will also have.
One of the best places to get heroes if from the chests in the store that cost gold. You can find characters fast rather than having to put then together with many fragments. Once you reach a team level of 10 you will unlock the ability to get fragments that can be used to acquire new playable characters.
At first your best team is going to be probably the first players you find. You might not want to level them up too much though encase you find someone who is better and then you would have resources to upgrade them.
You might have noticed already, but hero star levels only seem to go up to 7 instead of the 10 it looks like it should. This might mean that the developers could come out with some new content after some time. For now the best players are limited to only a couple with the rest of them having less stars. You can check out stats on heroes before obtaining them by tapping on the in the hero menu.
Again, if your looking for the best team then your going to want to get the players with the most power. Skills can also be a factor when it comes to choosing your party though the higher quality characters will most likely cause more damage over time then weaker units regardless of skills.
If your interested in more related information you can check out the Hero Hunters Wiki.