I think Halo 2 is the hardest of all these. (on legendary of course).
My answer comes purely from the fact that you will die alot more then in other games...
Even tough Half life is difficult it will take less deaths to beat it trough..
As with Halo people usually just run... die... spawn... run... die... spawn.....
Halo 2 is also nearly impossible on CO-OP... even harder then solo. (team mate dies... back to last spawn).
Also if you die 15 times, it will reset you back 2 checkpoints.
I wouldnt count guitar hero in this because it's a complete different gametype and cant be measured to halo.
Guitar hero takes time to learn, and it's more of an Reflex game. (like when you have to hit 5 notes in 5 second, you dont think, oh i'm going to hit that button next. Your brain automatically presses the buttons on reflex.)
There is also this one game (i think it was mech assault or something similiar)
It had a special "keyboard controller" with 2 joysticks and 50 buttons or so.
You had to beat the whole game trough without dieng, if you died it was game over dude!!
I think no one has beaten the game on the hardesd difficulty so far... On easy it's as hard as Halo on Heroic.
-Fin
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