When I ever have to go to CA I will always go to In & Out for a Double Double and fries. I have not found a burger in CO that is as good but then again, I don't really go out for burgers much anymore anyway. And I ain't gonna spend $10+ on a burger to find one as good as In & Out.

If I am out in the valley north of LA I will visit Tommy Burger for their Chili Cheese burger. Only the one near the Budweiser Brewery makes them right, the spin-offs just don't get it right for some reason.