What is the best handheld game for my 6 yrs old son?

theres alot... but there are diffrent systems... this question lacks specification of budget and or what type of handheld console your asking games about... psp has more mature titles then the ds... but the ds doesnt have such mature titles... the ds does have brain puzzle type games though... and educational... and alot of good titles rated e-t so i recomend the ds library of games... like pokemon,dr blah and the diabolical box... or something like that.