Smash Bros: Meet the requirements in unlocking a character, then beat them in a minigame (or a one stock match in the case of Smash Bros). If you lose, you have to meet the requirements again for another shot.
Mario Kart: Simply meet the requirements. The character will be unlocked afterwards.
Mario Party 6 and Mario Party 7: Buy the character from the game's bank.
Mario Party 8 and Mario Party 9: Beat the story mode once (or twice in the case of 8).
Super Smash Bros. Brawl and Mario Super Sluggers: Find the character in the story mode and he or she will be unlocked after beating the challenge given to you (or the level in Brawl's case).
I'll say either Smash Bros or Super Sluggers, but make the adventure mode a little more challenging and not too long.
Mario Kart: Simply meet the requirements. The character will be unlocked afterwards.
Mario Party 6 and Mario Party 7: Buy the character from the game's bank.
Mario Party 8 and Mario Party 9: Beat the story mode once (or twice in the case of 8).
Super Smash Bros. Brawl and Mario Super Sluggers: Find the character in the story mode and he or she will be unlocked after beating the challenge given to you (or the level in Brawl's case).
I'll say either Smash Bros or Super Sluggers, but make the adventure mode a little more challenging and not too long.