Play the first one. It's actually still my favorite in the series, despite the controls taking a little more getting used to.
It's a really great introduction to the characters and spending the entirety of the game on a single island really makes it feel like home. You'll grow to love the citizens as you follow each of their stories while learning every nook and cranny of the island. It's a great intimate journey that MML2 doesn't quite reach since you're actually globe trotting for the majority of it.
It's also decently short and sweet, while offering a ton of extra content if you're up for the task. Here's the
OT I made if you want a little more detail about what the first game offers.
Tron Bonne is a great compliment to both games. If you end up loving the characters, definitely pick it up. Here's the other
OT I made in case you want a better idea of what kind of game this is.
I recommend to play them in order of release, MML > MoTB > MML2.