Though most people would go for Ridge Racers first, I personally recommend Golf as a first game, as it's easy to get into, and though I've never really liked any golf game before, this one has sucked up hours and hours of my time. Easy to play in japanese, lots of courses and challenges and unlockables. Ridge Racers is great too it just didn't hold my interest as long.
I'd avoid NFS Rivals until it gets a proper review... read the previews at Gamespot.com... seems to me the game had some issues the couple times they got their hands on playables, major ones like f**ked control. I'm personally going to wait for GT4 Mobile unless NFS gets a GREAT review.
Latest word on GT4 mobile is it will be more or less an exact PORT of the PS2 version (minus a hit in the graphics, of course), including the ability to transfer saves to and fro between the two... GT4 for PS2 releases TOMMOROW f'ing SWEET!!!