I have to say Vegas 2.
I know Vegas 1 was so good that Vegas 2 is almost disappointing, but I love the ACE system, loving every last bit of it.
If it weren't for the ACE, I'd always playing as a markman style. Hide behind cover, come out and aim for headshot and all. But because of the ACE, I even use blind shots when in 1:1 situation just because blind shot gives +3 EXP. Also I started using explosives more often.
It realy jazzed up my playstyle, and I'm grateful.