Hmm very hard, none of them is really my style but if i have to choose i would say the last one is the best .
im gonne have to go with the dude above me... the last 3 i think are too bright for my liking. the first one, if the group of people is your focal, you need to blur the guy in the very front left...and slightly blur the stuff on the right.. depth of field, learn it and you will be leeeet [:
first is extremly hot. I dislike the flatness of Einstein's one but unusual render and probably LQ stock at start (since he's mad old..!), so yeah, 3rd one is strange. Especially left girl's face. Like the blur effect on the right side but dislike the text. The last one is one of the best outcome with this whored render so far, except that I dislike those white splash.