The US constitution is the oldest, still working, foundation for a government in the world today. Every other Democracy is either younger or has gone through one or more complete changes to theirs since their inception. And every non-Democracy has gone through more extreme political changes still. To say that the American system of government is broken... indicates what exactly for the rest of the world?
America makes alot of mistakes and missteps, it abuses power, doesn't properly handle many (or even possibly most) situations as it "should." But let's face it, like Winston Churchill said "Democracy is the worst form of government except all the others that have been tried."
The US's system was not originally designed to be a 2 party system, that evolved later. However, it's overall design has let it keep working roughly as intended, with not a single interuption in service (aka no coups no, shutdowns- and I don't mean that budget issue we had the other year I mean a real shutdown-) That's saying something given how many internal and external wars, depresssions, and social revolutions it has been through. For all the faults it has, the US political system survives because it works. It's not fast to change or adapt, it makes lots of mistakes, and it can be manipulated, but the same can be said for a whole lot of other goverments out there *cough-all-of-them-cough* only without so much stability. More parties introduce instability, they force coalitions in government which can lead to shutdowns and standoffs that can become crippling.
Meanwhile the US two party system (which is not legislated in any way, just "encouraged" (it could change, just no parties have manage to rise to challenge it)) is stagnant in many ways but is extremely stable and reliable. What's best? That question must be answered entirely based on your personal experiences and views of the world.