With Matt Hughes it really depends on the draw, I think he Couture, and maybe Mark Coleman are still in real fighting shape. I would take Hughes over Liddell, Ortiz, Shamrock and Gracie.
Rd 1 would likely be split by weight not age or likely rankings
Dan Severn (HW) vs Randy Couture (HW)Couture via TKO 2
Mark Coleman(LHW) vs Chuck Liddell (LHW) Coleman via TKO 1
Royce Gracie (MW) vs Matt Hughes (WW) Hughes via arm bar
Tito Ortiz(LHW/MW) vs Ken Shamrock (LHW/MW) Ortiz via TKO 2
Couture vs Coleman Couture Dec
Hughes vs Ortiz Hughes TKO
Couture vs Hughes Couture TKO 1
Now if you go by how they would likely be ranked
Randy Couture(1) vs Ken Shamrock(8)
Mark Coleman (2) vs Royce Gracie (7)
Matt Hughes (3) vs Dan Severn (6)
Tito Ortiz (4) vs Chuck Liddell (5)
Then I would see it Couture, Coleman, Severn, Liddell
and then Couture, Coleman and then Couture.