It's a shame to see how Solar Crown turned out. I put a metric shit ton of hours in Test Drive Unlimited (and TDU2 to a lesser extent) and absolutely fell in love with the series. The driving mechanics have always just been okay, but it was great as a collective package as an auto enthusiast lifestyle game. The atmosphere and detail of the dealer showrooms is one of those things I am amazed games like Forza or GR haven't adopted. It was cool being able to customize your cars down to interior options and it made the process of getting a new car more personal. The progression and money flow were paced well enough that you didn't have to grind a ton, but still felt satisfied when you saved up enough for a new high-end car or a new property to customize and show off cars in. The map size is actually larger than most Forza Horizon games (all of them by a large margin in TDU2's case with Hawaii and Ibiza). Even though the map isn't super dense with points of interest, the variety in scenery combined with the sheer scale are great for cruising. The game even has a fully rendered cockpit view, the ability to roll down your windows, and retract your convertible top (a "new feature" in FH5). You can even cut off the engines, which have a unique quirk of having different startup times based on the car's era (i.e. older cars take slightly longer to start). To top it off, my favorite part, every vehicle has gauges that accurately recreate the visual style of the car. There's just so much attention to detail for the people who truly love cars.
The Xbox 360 and PC versions are a bit hollow given how their online services have been down for a while. The PS2 port of TDU, however, is a complete single player experience that only sacrifices some graphical fidelity and a couple side activities, leaving the car roster and map scale intact. For a 20 year old game, it has aged remarkably well. The racing "MMO" concept was pretty ambitious for the time and laid the groundwork for the Forza Horizion series.