I might certainly be affected by childhood nostalgia, but NFS Underground and Underground 2 were my favorite games in the series.
Underground had the perfect balance of difficulty and arcade fun, adding many new innovations like drag racing, drifting and of course, nitrous oxide! It was a big leap from its predecessors, which back then largely only featured simple racing. The fact that you could customise the look and performance of cars was an added bonus.
In my opinion the formula was almost perfect. Underground 2 did add open world exploration, and several new customisation features and race modes, but Undergound was the most amazing one for me. It would certainly be hard for NFS to create a better game in the series, but I would definitely pick it up if they did.