The Mario & Luigi series as well as Paper Mario aren't what I'd call 'spinoffs.'

That being said, I am not inclined to type a huge fucking post about this, believe it or not, so I'll keep it brief:

Play SMB1 for novelty's sake. Respect the roots.
Play SMB: Lost Levels (Don't start with me about the name.) if you want to get your ass reamed by a Mario game.
Play SMB3 because it is utterly iconic.
Play SMW and Yoshi's Island because YOSHI, FUCKER.
Play SMRPG because it's full of so many minigames and fun Easter eggs and tidbits while still being faithful to both Mario and RPG fans, and then play its spiritual sequel, Paper Mario.
Play SM64 for the same reason as SMB3.
Play SMG because it is absolutely beautiful.
Play SMG2 for the same reasons as SMG and SMB:LL.
Play any of their sequels if you liked the first ones.
Play the spinoffs if you like racing, sports, and/or minigames.
If you like certain characters, give their games a spin. Luigi, Donkey Kong, Wario, Peach, and even Toad have their own games spanning many consoles and a few genres.
And whatever you do, DON'T play Mario & Luigi: Partners in Time. We don't discuss that game.

That is all.