I have voted for The Office up until this point but this is as far as it is likely to go.
How I Met Your Mother is a show that I had known very little about until about a year ago. I caught the very last part of it on a stream whilst waiting for Raw to come on and I fell in love. Barney Stinson is a character that every man aspires to be and since then Neil Patrick Harris has been a hero to me. HIMYM is a fantastically brilliant show and it's humour may only be loved by few but it is very infectious. The cast are absolutely brilliant and whilst I still cannot name them all first hand, I know that they put on a great show every week. Jason Segal and Allyson Hannigan are brilliant as a couple and this show really launched Segal's career in comedy. However, as I said, Neil Patrick Harris is the life-bloof of this show and I could not imagine it without him.
Sure, The Office has some good characters and Ricky Gervais is a good actor. I enjoyed The Office during it's run and it is of no wonder that it has gathered cult acclaim in the UK and beyond. However, If I was forced to choose between the two, it would be How I Met Your Mother all day long.