Attach rate is completely the wrong approach here IMHO. That is looking to the importance of that game to the console, but I think if you're talking 'bigger' then raw sales units is the measure to use.
A console could sell 10M units and have a game so popular with the demographic for that console that it sells 8M on the console - wow an attach rate of 80%
Another console could sell 100M units and have a game that sells 20 M units - gee, only a 20% attach rate.
However, the former game is obviously more relevant for the first console, however, with an additional 12M in sales, the second game is clearly the more popular overall - or bigger franchise - as it reaches more individual people.
Halo is a big franchise, and for Xbox its a massive franchise - so important I believe both the Xbox and 360 couldn't have survived without it.
However, a franchise like GTA is clearly more popular as its total unit sales far exceed Halo, for example. And the Mario franchise is clearly much larger than Halo.
As for something like GT, well, I think total sales of all games to date are probably higher than Halo, but then there have been more GT games (GTA too for that matter).
Then there's the complication of exclusive vs cross platform. Halo is exclusive, as is GT, but GTA is not.
In the end there's lots of ways to play with the numbers, but what I think everyone is fumbling around here is the following:
1) no Halo isn't the biggest game franchise in history
2) yes, it is by far the biggest franchise on Xbox and the most important
3) yes, there are other exclusives that are larger franchises, but only a few, Halo's in the top ranks too so relax
4) yes, one of those franchises (GT) is on PS3 - just relax, Forza's great too, and maybe over time Halo will overtake it (after all it trails in games released currently)
Try to be reasonable... its easier than you think...
