While Bowser's Inside Story is the fan favorite and most likely the one you'll enjoy the most, I recommend starting with Superstar Saga if you don't plan on playing the 3DS remake, because those QoL improvements that were mentioned already? Yeah, that also applies to future titles after Superstar Saga. I also think that Superstar Saga is a good "baseline" for what to expect from the franchise (unlike Bowser's Inside Story, which is an outstanding title for the series), so it's a good entry to determine if you'll like any of the others after it. I also recommend that if you plan on playing Partners in Time, you do that before Bowser's Inside Story, as BIS makes a direct reference to through some side content (like, as an actual optional segment of the game) that you won't be able to appreciate if you don't play that first.
I recommend against playing Paper Jam first. That game in particular has features no other game before it has, and you will be sorely missing them. While Dream Team is my personal favorite, I also think its story hits harder if you play at least BIS beforehand.
Honestly, you can't go wrong going with the order that they were produced in. While you will completely understand the story if you play them out of order (none of them hinge on you playing the others first to get it as the stories are self contained), certain events won't hit quite the same way if you don't play the game that comes before first. In addition to that, the changes that are made to the gameplay can make the older titles harder to go back to.
