Bill Clinton - Bill Clinton attended a service in October 2000 in memory of the 17 sailors killed in the attack on the USS Cole.
After the terrorist bombing the Murrah building in downtown Oklahoma City he publicly grieved with the families of the victims at an event that was regarded at the time as a turning point in his presidency.
Lyndon Baines Johnson - According to the Johnson Library, LBJ attended two funerals for soldiers who died during the Vietnam War. The first funeral was for Captain Albert Smith, son of White House correspondent Merriman Smith, which was held February 28, 1966. The second was for Major General Keith R. Ware, held September 17, 1968. LBJ had met Ware while visiting Vietnam.
Richard Nixon - Richard Nixon does not appear to have attended the funerals of any soldiers killed in Vietnam.
George H.W. Bush - President George Herbert Walker Bush does not appear to have attended any funerals for American soldiers. (The NYT, citing Marlin Fitzwater as a source, indicated that the president did attend several such funerals. But no details were provided.)
http://blogcritics.org/archives/2004/09/14/150821.php