I hadnt read OP's post (only the title) when I decided what my guilty treasure would be, but coincidentally its another Lego game.
Mine is Lego Racers for PC. It's a simple but solid kart racing game that just felt good. It had me addicted for way longer than it had any right to. If I had known as tween what speedrunning was...lets just say my life could have been very different (referring to the fame and riches I could have accrued from my mastery of this game). It had really satisfying power-ups and shortcuts. There were a few bosses with their own themed circuits, and a car/character creator where you could mix and match themed pieces from the boss cars after you beat them.
To be honest, I'm not well enough in-the-know to about "little known/covered" it is/was, but its not a game I've seen or heard of many people playing, so hopefully it counts. I would still recommend this game today to anyone who enjoys arcadey racing games.