non opaque primary topics

the standard model in sem web seems to be

<> foaf : primaryTopic <#relative-uri>

I've seen 4 relatively common relative URIs used here and there

1. #me -- common in foafs
2. #i -- often used by timbl and tobyink
3. #this -- used by kingsley as per the 'this' keyword in OO
4. # -- used by facebook and others

It's a very common case that a document will contain one primary topic and
I want to standarize all of my pages (mainly robots) into one of these 4.

It seems to me that (4) is the most sensible choice, after "it doesnt
matter".

Does anyone have any preference here?