I think that the best form of a democracy is to have it be as open as possible. Primaries really are no different in this regard. We want everything to be as open as possible, so that the best candidate may be chosen to be the one to lead the party.
Yes, I think that open primaries are the most effective method of selecting candidates for elections. This is how primaries are conducted in my state, and I think that it works. This way, voters can simply choose the candidate they want rather than having to register with a particular party.
Like other posters have said, an open primary may end up making it harder for small parties and minority voices in large parties to be heard. If one party runs a large number of candidates, they will hurt their own interests, so small candidates will be forced out. The way primaries were originally organized was designed so small voices can be heard.
In an open primary the top two candidates go on to the general election. There is a real danger that if one party is simply more divided than the other party in a place with open primaries then that party will lose elections in the primary automatically. If one party is dividing its vote between five candidates but the other party only runs two candidates, then in the general election voters could have a choice between two politicians with the same exact positions. Eventually the two parties will have to make sure that only one or two candidates from their side runs IN THE PRIMARY, which will make it so minority voices are even more stifled.
Open primaries are better than closed primaries, but I wouldn't call it the best system. A better system would be where political parties were banned and where candidates would be limited to campaigning in an enclosed venue, maybe a website or a public forum. Campaigning outside this venue would be banned so the wealthy wouldn't have an advantage over the poor.