per the research I have read, probably the best bang for your buck is Source Naturals - Life Force (available with or without iron).
the serving is 4 caps, but i never take more than one, twice per day (different meals).
centrum and one a day both suck. any hard, compacted and coated vitamin is not being optimally absorbed. Life Force is a powered/gel capped delivery for better absorption (which was proven with research).
I get mine at