You're kidding, right? nGage and Gizmondo and a host of others push Vita well off the top-spot for biggest handheld failure.
I meant it on this gen. Historically, there are some heavy weights in there for sure. Even nintendo joined the fight with virtual boy... and gbmicro to some extent.
Making a successful dedicated gaming handheld was hard enough before smartphones came to the yard, you have to be nearly lunatic to attempt it now.