Waku Waku Puyo Puyo Dungeon is a lot of fun, and easy to play even if you don't know any Japanese.
It should have been released in 1989 (as a Mega Drive launch title or something) instead of 1998 (last Saturn title ever) though.. That game is SO after its time.
Puyo Puyo Dungeon is hardly the last Saturn game. It's not even the last Puyo Puyo RPG game on Saturn.

Madou Monogatari for Saturn came out in after Puyo Puyo Dungeon in 1998. It plays more like a regular console RPG though. Heck, the last Saturn game didn't come out until 2000 or 2001. Can't remember if it was Final Fight Revenge or a multi-disc adventure game whose name escapes me right now.
BTW, there's a port of the first Madou Monogatari game for PC Engine. I haven't been able to get a copy yet since it comes up rarely on ePay, and the price is always more than I'm willing to pay.