I could go on at length about this but here is not the proper forum (former life = education consultant in the area of language acquisition (LA), and LA delays).
So, in short: As function of the text is critical to what you are doing, your best bet is to provide some sort of emphasis to the text without interfering with its legibility. The simplest way to do this is to capitalize the first letter of each word, as in your first example.
In fact, your current order of examples is pretty much in the order of best to worst options.
Obviously the graphic design for the page must also be taken into account - but we don't have that here, so my opinion is based purely on what will readers most quickly comprehend.
(The initial cap only approach seems to be getting more popular - e.g., The title - which is a pity. Western readers most readily associate this appearance with the beginning of a sentence and so expect to see more text immediately follow. When more text does not follow - as in your case - it causes a momentary pause, an adjustment. It takes only milliseconds, but it does occur, and I would recommend avoiding it if possible.)
Note that Rahul's non-example would be, in almost every instance, the worst decision you could make.