I definitely think the best looking GT40 is the Mark II rather than the Mk I or IV, although I do like the Mk IV for being completely different, and to me it looks like if it had slightly more aero it would fit into the 70′s quite well, whereas the others look to me more like 60′s prototypes (which or course they are) or later GT cars (although this impression might come from the Ford GT being created as a road car).
But I do prefer the more muscular Mk II. But I think all GT40s are very effected by livery.
http://www.google.co.uk/imgres?q=gt40+mki&um=1&hl=en&safe=off&sa=N&biw=1366&bih=667&tbm=isch&tbnid=ecfO5ytBRF_ZPM:&imgrefurl=http://obviousmag.org/en/archives/2009/03/ford_gt40_-_the_eternal_sports_car.html&docid=IG8GoL20uF0iJM&imgurl=http://blog.uncovering.org/archives/uploads/2007/070823_blog.uncovering.org_ford-gt40-mk-ii_1.jpg&w=518&h=397&ei=ffaaTtrUNMu68gOwsdXgBQ&zoom=1&iact=hc&vpx=382&vpy=324&dur=1122&hovh=196&hovw=257&tx=162&ty=109&sig=111404202231557491931&page=3&tbnh=140&tbnw=183&start=33&ndsp=18&ved=1t:429,r:7,s:33
These 2 look pretty awful to me, worse than a Mk I in a decent livery.