[Ekcrbe]

You can always say that a well-executed swerve drive by a team that knows both how to build it and how to get the most out of it with programming could be the most effective. That being said, the most effective drivetrain is often the one your team can drive the best and gets you where you want to go despite your opposition. The drivetrain you can drive best is the one you can practice with the most, and getting where you want to go usually means having good traction, either to run through defense or to play it.

[aside]
Yes, we can debate about whether mecanum/holonomic allows you to juke out your defenders, making traction irrelevant, but suffice it to say that finding this level of skill and control in a driver and robot, if it exists, is rare.
[/aside]

There are a select few teams that have the capacity to build a complex drivetrain fast and train their drivers to a high degree of skill with it, but for everyone else, stick to a 4/6/8-wheel skid steer and it will be the most effective for you. To fulfill Andrew's definitions, a skid steer will be the most effective drivetrain choice for most teams, and as such it will be the most successful drivetrain on aggregate across FRC.