I think it is the 4x400m male world record. Andrew Valmon, Quincy Watts, Butch Reynolds, Michael Johnson joined to run a mind blowing 2:54.29 back in 1993. Michael Johnson, Tyree Washington, Jerome Young and Antonio Pettigrew however did run a 2:54.20 in 1998 but this was scrapped due to doping during the time by Antonio Pettigrew. However my point remains and these times are absolutely monstrous and It would be insanely hard to get this many good runners on one team now.