Frank Mir or Brock Lesnar in a fight?
Sadly, I think Brock Lesnar would trash Frank Mir 9 out of 10 times.
There is no way Frank Mir is going to knock out Brock Lesnar, considering how Lesnar took Carwin's devastating punches and still survive pretty clear headed. The only way Mir could win is through catching Lesnar with some submission which I think is pretty highly unlikely as if you look at the second fight between them, Lesnar was pretty well prepared, actually displaying some legitimate skills in BJJ to prevent Mir from getting up.
Frank Mir is also a douche. He says he would win Lesnar 99 out of 100 times? WTF? And also he wants to kill Lesnar in their next fight. Now, who's acting more like his in the WWE?
All these makes for a great rivalry but its pretty one sided, Lesnar is just too much of a freak athlete and with skill too, so sadly for Mir he would just dominate MMA for a pretty long time.