1996's Super Mario RPG remains a fan favorite even today, with a lot of attachment from fans; it was one of the last RPGs released by Square before they switched platforms and primarily developed games for the Playstation platform. Because of this, Nintendo's plans for a Square-produced sequel fell through, and they were forced to rely on their second party developers instead. Intelligent Systems produced the Paper Mario series beginning with the Nintendo 64, while several years later they would commission AlphaDream to produce a second series, starting first on the Game Boy Advance with "Mario & Luigi: Superstar Saga".
Both games have a heavy focus on minigame-driven mechanics and Mario's long-established trademark of platforming and puzzle-solving. Superstar Saga definitely takes a more novel approach, though, with some inventive mechanics both in battle and out. In-battle, you control Mario and Luigi in a turn-based combat style, with timed button presses being utilized both to deal extra damage when landing hits on enemies, as well as mitigating or avoiding enemy attacks when they come your way. Each brother is mapped two a face button (A or B) and commands are executed solely with that button for that brother - something to keep in mind when an enemy turn comes and you have to press the right button to jump and avoid an attack. Each enemy has a tell for whom they're about to attack, though - for example, they will speak in red or green text, or throw spiked balls with their left or right arm to telegraph whom they're attacking, giving you a moment to prepare and counter the move.
Special attacks are based around timed button presses as well, and these make way to one of the game's most memorable mechanics - Bros. Attacks. Basically, Mario and Luigi spend BP (Bros. Points) do a tandem attack against the enemy, and at certain points in the animation you'll be prompted to make a timed button press to prolong the attack. Doing all the attacks in a sequence successfully will result in an elaborate and flashy move that deals considerable damage, while missing it will play a shorter but often humorous animation of them botching the attack (for example, one brother will trip and send them both flying into the enemy); it still does some damage, but not as much as if you'd successfully timed the press (though the 'botch' animations are worth seeing at least once regardless). You're also given the opportunity to boost the "challenge" of performing the move to potentially bolster its damage even further - challenge level 2 removes the slow motion window before the button press, while 3 will remove the prompt showing what button you're supposed to press (though that should be a minimal concern once you've done the move enough times).
Mario and Luigi work in tandem in combat, but also outside of battle as well; the game is loaded to the brim with puzzles that require them both to interact in a lot of clever ways. There are coin blocks that move back and forth, requiring you to position both brothers and hit them back and forth to get maximum coins, but you also get a variety of abilities to traverse obstacles. Luigi can use Mario as a platform to bounce them both to a higher ledge, Mario can use Luigi to spin and fly over gaps, and Luigi can flatten Mario with his hammer, letting him slip into narrow gaps he couldn't reach while full-sized, to name a few. These get used liberally to traverse the game's many maps, and also add greatly to the slapstick humor present throughout the game.
Of course, the game is still an RPG at its core - using items to restore HP and BP, gaining experience and levels and upgrading equipment with money remain key elements of the experience. Even these get their own flourishes, though - some gear will add beneficial effects, like causing enemies to always drop items when defeated, guaranteeing you get the first move in combat, or regenerating a small amount of HP every turn, so strategy doesn't just boil down to which piece of gear gives the best numbers. As in Mario RPG, gaining levels isn't a fully straightforward process, as you're also given the choice of one additional stat to grant a semi-random bonus to, letting you shape your party's stats to a degree (though if you're a die-hard min-maxer, this may also result in a lot of tedious reloading to get the maximum bonuses each time). Using items mid-battle further shows off the game's polish - rather than simply having a character toss an item into the air and have it take effect, it will actually show an animation of them eating or drinking it (or, if reviving a downed character, they will stuff the 1-UP mushroom into their mouth to have it take effect).
Superstar Saga is another attempt by Nintendo to recapture the charm of Super Mario RPG, and I think it honestly does an admirable job; even moreso than Paper Mario in some ways. The fundamental gameplay of Mario-style platforming with RPG elements and minigames is represented quite well here, and the expressive characters and polished animations (especially for the Game Boy Advance) fit in perfectly with the Mario series. The manic sense of humor throughout, with characters constantly having hilarious dialog and showing exaggerated stretch-and-squash animations, make it quite a funny game to experience, outshining even many of the officially licensed Looney Tunes games in that regard. It may not have quite the same style and high-quality presentation as Square's RPGs, nor the in-depth customization of its cousin Paper Mario, but Superstar Saga is another worthy Mario-branded RPG that proves to be quite fun with its madcap style and inventive game design.
Platform: Game Boy Advance, Wii U (Virtual Console)
Recommended Version: The Wii U VC version is a direct emulation of the GBA original. The game also had a remake on the 3DS, which actually does a pretty good job capturing the stretch-and-squash style of animation despite the leap to 3D-rendered graphics over sprites. The original's hand-drawn animation and 2D graphics still hold more charm in my book, though.