UG was good, UG2 was absolutely fantastic... then Most Wanted and Carbon basically copied the same formula with variations, but didn't pay as much attention to level design as UG2, which was the best of the "free-roam" PS2 titles.
Never played undercover, though... after the disappointment of the "more of the same" from MW and Carbon, and the utterly ridiculous "Prostreet" (If I want to drive on the track, I'll do it in GT, thanks...), I couldn't be bothered.
UG2 will hit the spot... but if you're coming to it just now, the graphics are somewhat dated, and Carbon has better customization... but the whole city of UG2 is a free-roam paradise, with lots of instant battles available.
I will be picking up Shift.