For me the most effective one has been following my breath. The principle there is that nothing can get more real than your breath right now. You are , only if you breathe. ( or atleast it resonates a ton with me ).

If thoughts come into my head I classify them into 2 - thoughts or feelings? Pleasant or unpleasant? and continue focussing on breathing. I do the above 15 mins everyday. This , for users of headspace, will probably be well known.