Sin & Punishment: Successor to the Earth. Though I technically had played Wario World on the gamecube, S&P felt more like my first real taste of what Treasure was really all about when it got re released on the Wii Virtual Console. Always heard the name thrown around in terms of notable N64 games from late in its life cycle, but I never knew much beyond that what it even was. Needless to say, it's my favorite from not just Treasure but for the system as well. Even in 2007, it kinda blew my mind in a way that I did not think something could've been made for that system. The cutscene direction and nonsensical anime bullshit, combined with the rapid variety of boss fights and set pieces. Act 2-2 and 3-3, man.
It was fun, and crazy, and most importantly it's short enough and well paced to be cleared in one sitting. Something that I feel has become a bit of a deal breaker for me with the sequel despite it being even better in every other regard. Just that the total run time is too long for a game that sticks with such an arcade design philosophy and encouraging you to get the best score. 45 minutes is more palatable to me than something that runs just over 2 hours, and this is not counting the cutscene run time in both games.
I wonder how many people have played their entire catalog. They were a staple on Sega Channel back in the day. I played so much Dynamite Headdy that way.
At this point, I've touched at least all of Treasure's games that aren't based on an existing license. I keep thinking about trying that Astro Boy game they did on the GBA as I hear it's particularly good. But otherwise, their original stuff has always been the main draw as they always sold me on how they focused on creating their own spins on arcadey games that are more to my liking than the things that inspired them (Guardian Heroes is arguably my favorite beat-em-up ever and it's made me very particular about how I like that genre).