Racing games are by far my favorite genre, I don't think I could pick an absolute favorite, so I'm just gonna ramble for a bit.

Need for Speed Hot Pursuit 2 through Carbon are fondly remembered for a reason, they still stand up so well today. Outside of the golden era of NFS, Hot Pursuit (2010), The Run, Most Wanted (2012), Heat, and ~~Unbound~~ (absolute trash update today, nevermind on Unbound) are very good as well. Other games in the series are good too (I'm a strong defender of Rivals despite it being objectively subpar), but I'd consider these to be the highlights.

The Burnout series is excellent, especially Takedown, Revenge, and Paradise.

Forza Motorsport 4 and Forza Horizon 1 are the absolute peak of that franchise, absolutely beautiful games, I love the atmosphere and career pacing.

The PGR series is great as well, and only got better with every title. PGR4 (the last one) is amazing. This team also developed Blur before shutting down, basically Mario Kart with real cars, which is massively underrated.

The Flatout games (1 and 2, we don't talk about 3 and 4 is eh) are great. Bugbear also developed Wreckfest, which is a spiritual successor that improves on the original in almost every way.

Driver: San Francisco is one of the greatest games ever made, and I'll never forgive Ubisoft for abandoning the series.

The Motorstorm games are great, and Evolution Studios also worked on the excellent Driveclub before being bought and merged into Codemasters (and later EA...)

Enthusia is another super underrated one, really unique progression system that encourages you to drive interesting cars instead of the fastest.

The DiRT and GRID franchises have some great entries as well, especially DiRT 2/3 and GRID 1/2. Codemasters also developed FUEL, which I think held the title of biggest video game world for a long time (it might still if you don't count stuff like Flight Simulator).

Split/Second is excellent despite its shitty PC port.

There's a bunch of great indies as well, Distance, Redout, Inertial Drift, Horizon Chase Turbo.

There are lots more, Midnight Club, Ridge Racer, Gran Turismo, Test Drive, but this comment is way too long as-is... It is also fairly depressing that most of these are dead franchises, and/or aren't available to purchase anymore due to licensing expiration.