It's going to be close because there is no clear favorite. The only one I'd consider going back to play is Mario Tennis, because I never played it and loved my time with Mario Golf GBA last year.
My friend got really into Custom Robo DS last year, but I don't think it's for me, and I loved River City Ransom when I played it when I was younger, but I don't have any motivation to back and check it out again.