Marioverse without a doubt.
Rosters would be like:
First game
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Includes the Big 8 Mario crew members(Mario, Luigi, Peach, Toad, Yoshi, Wario, DK and Bowser) as the starting roster.
Secret characters are Smash style and a little more surprising than what you'd expect with Jr being the only kind of obvious. Kamek, Wart and Fawful all get in as additional villains representing more Yoshi-verse, an obscure forgotten villain and a RPG villain.
Second game
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Adds in some vital missing characters; Toadette, Daisy, Waluigi, Rosalina and Diddy Kong.
More additional surprises with two SMRPG partners in Mallow and Geno, two Paper Mario the Thousand Year Door partners in Goombella and Vivian, some additional Wario-verse characters with Captain Syrup and Tatanga returning, Nabbit of the New Super Mario Bros. Wii U fame and King Bob-Omb as a surprise character added as he's the first major boss in Super Mario 64.
Third game
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Doing the Double Dash thing and partnering the characters up. Yoshi obtains Birdo as a partner(as is tradition), King Boo and Petey team up(as they did in MK:Double Dash), Dixie partners up with Funky, Cranky(the original DK) partners up with Pauline, Toadsworth and Prof. E Gadd team up as a pair of cool helpful grandpas and finally Ludwig and Topper as the two eldest of their respective villain troupes get paired too.
Fourth game
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Throw in the rest of the Koopalings so Ludwig isn't all alone, gave Topper Hariet to hang with too. Three new Warioware characters are added in Ashley(who is ridiculously popular), Mona(also very popular), 9-Volt(retro game theme moveset). K. Rool and Lanky join the Kong universe. Peach gets two new flavours in Peachette and Rabbid Peach, Captain Toad makes the roster, Paper Mario gets General Guy, Mimi and Dimentio, Luigi's Mansion gets Polterpup and Yoshi gets to hang with fellow dinos Plessie and his rival Boshi(hope it makes
Zinith
pleased). Il Piantissimo is a lethal joke character.
Kinda sad I couldn't find even fan 3D renders for the Warioware cast but ah well.