To the OP: All AR-15's are "based" off either the M16 or M4 platform, and all of them have roughly 100% parts commonality within the M16 or M4 subcategory. The reason folks keep talking about the Colt LE6920 is because that is the standard by which all others get measured, that is not to say it is the absolute best, but it's the standard. You may also have heard the term "milspec", that is tossed around to describe the quality of the parts used, particularly the bolt carrier group and barrel. By quality I mean the steel they are made off, testing methods, and many other particulars. What you need to read is the "Chart", which is an AR guide about what to look for and what popular companies put in their AR's, it can be found on M4carbine.net. Just google search some keywords.
Popular brands that generally meet the basic standards are:
Colt, BCM, Spikes, DD, Stag, Olympic (I believe they do), PSA, LMT, and others. The little I have heard about Windham Weaponry suggests they are pretty good as well, better than their old owner Bushmaster.
I am by no means an expert at all, but a relatively short time ago I went through the same process of figuring out what the heck everyone was talking about when AR's were being discussed. I currently have a LE6920, and love it.