Least competent: Iraq 1991 (sort of an obvious choice)
Most overrated: Ho Chi Minh - Got a lot of the credit that belonged to:
Most underrated: Vo Nguyen Giap - Understood, long before the Americans even began to consider it, that all he had to do was NOT LOSE. Almost blew it once or twice (Da Nang possibly his biggest error) but won.
I actually think that Peng Dehuai was the most under-rated military general that the US faced. The Chinese pushed the US back from the Yalu river to Seoul, forced Truman to declare a state of emergency and broke MacArthur in the process as he threatened to go rouge/nuclear. Peng did this with Mao breathing down his neck, with mostly only infantry (though highly experienced infantry) with little to no aircover, few artillery pieces and terrible logistics.