Need for Speed: Underground 3

Most Wanted (2005) could be considered Underground 3 and Carbon (2006) could be considered Underground 4. In the Prima guide for Underground 2, they interviewed Marc DeVellis, who started at EA with Underground 1. Marc mentions Underground 3, but that turned out to be Most Wanted (2005). My impression is that most of the core team at EA Black Box that developed Underground and Underground 2, continued on with Most Wanted, Carbon, and perhaps Undercover. EA Black Box under a new name, also worked on NFS World and The Run, but I don't know how many of the original development team was left by then. After The Run,  EA started laying off the team, which mostly impacted NFS World, the team was down to a small support group by April 2013 and World was shut down in July 2015.

For Underground 2, the Toyota Corolla, a starter car, is the fastest circuit type car due to the high cornering grip the game gives it; the fastest drag car 240SX, highest scoring drift car Golf (even though real drifting events don't allow front wheel drive cars), the fastest SUV is a tie between Escolade and Navigator.

Interest in tuner cars had diminished by 2004-2005, so EA included the exotic cars that were a part of the NFS games from the 1990's starting with Most Wanted.

Underground 2 was popular partly because it was the first NFS with free roam, and having 6 racing modes (circuit, sprint, street cross, underground racing league, drag, drift). Most Wanted was the most popular NFS game of the series, partly because consoles were dividing the player base, which affected Carbon and later games. Interest in online racing games in general probably peaked in 2005 / 2006, with the exception of NFS World which was free to play (but pay to win). 

I still occasionally play the old NFS games offline, and a few years ago, I was playing Underground 2 using it's LAN mode with a VPN (Virtual Private Network) for online play with other players. This is still possible, but I don't know what groups are supporting virtual servers for the NFS games with LAN mode. Carbon and Undercover online servers are still running.

Trivia - the first NFS with a storyline is Porsche Unleashed (2000), with it's factory driver mode.