I would consider Great Britain to be the originator of western democracy. The earliest roots of this are in the English Bill of Rights in 1689. Admittedly it wasn't initially what we would expect from a democracy but access improved over time. When America was founded the version of democracy was undoubtedly a significant improvement but it would have been influenced by the British model.

Over time other nations have created arguably better versions (proportional representation for example) and of course the US and Britain have improved their system as well by allowing more people to vote.

Have a look at The Economist 'Democracy Index' you'll see that the US currently ranks 17th of all countries http://en.wikipedia.org/wiki/Democracy_Index.

I personally believe the Parliamentary system originated in the UK is more robust than the US's presidential system. In a parlimentary system the prime minister is immediately accountable to the parliment. Their party (or a majority of parliament) can chose to dump them at any time which means that you are less likely to be stuck with some wack job in power for 4 years.

I alse believe the lack of preferential voting makes it virtually impossible for any other party to gain significant traction or become president. The only choice is Rep, Dem or waste your vote. This effectively denies genuine representation to people who's views are different from those a both parties. The preferential voting system is resonable for this but in my view proportional representation is the curent 'state of the art' in democracy.