I would say Gran Turismo 5 and I have a reason besides GT being my favourite series.
To the average gamer like me Halo Reach doesn't sound like a full Halo game. Now while I've played all the Halo games to some extent I'm not a hardcore Halo fan so I don't know the whole story, but just by looking at the titles it doesn't sound like a full fledged next installment.
I mean you've got Halo: CE (usually just called Halo 1), Halo 2, Halo 3...then when it gets confusing for a non-hardcore Halo fan is you have Halo Wars and Halo 3 ODST and when you throw in Halo Reach it sounds like just another spinoff and not a full sequel like I've heard it is.
On the other hand the next Gran Turismo is a perfectly title Gran Turismo 5. You know right away its a sequel (or next installment) right after Gran Turismo 4 and not a spinoff or anything.
That's just my 2 cents.