grafpt
What kind of customizing do you mean? That's too broad lol
Lol. Well, any type of customization really.
But two games stick out in my mind as top notch in this department:
Tracks: Mod-Nation Racers; the customization: TRUE TRACK CREATION (landscape type, surface specific elevation, surface type, placement of a variety of objects... I know I am forgetting some its been years lol) and the ability to share your tracks in this game are superior to any other that I have played. Yes I know what you are thinking: "Mod-Nation Racers..... Really?" And granted, I will give you that, I do not even own this game, rather had a fun couple of hours with it. But apply those same concepts to sim-type racing games and... well there is your winner. Just play it once; it is a far war-cry from the track RANDOMIZER that is included in some games..... ahem Gran Tur.... Ahem.
Cars: Forza 3; unless you have played it you have no idea what kind of customization options are available in this game and what they are really capable of. Top notch, high detail community created liveries, racing replicas, decals, and vinyl kits ranging from Monster Energy to Repsol Replicas to high detail anime drifting vinyl kits. Almost anything you can think of you can download it off of the storefront (an in game community store). Cant find it? You can create it with the in game vinyl editor, then upload it to the storefront and sell it for in game credits to purchase vehicles from the dealership or community auction house, download additional liveries, and purchase vehicle tunes. All available within the store front. You can also dyno your vehicle after each upgrade. Want to paint your car a specific color with a preview of the car after paint? Well, no paint chips needed here, there is a color wheel and full preview menu. Want to tint your windows? Nice. How about a Gemballa body kit for that Porsche Panamera? That looks amazing. I could go on... I can't wait for Forza 4.
Dirt 3 is more than enough to hold me over.
Can you tell which sim left me unsatisfied?
But ya that type of customization, sorry for the novel.