I really like Partners in Time, but compared to the rest of the series it easily ranks the lowest for me.
Anyway, I already praised the M&L series' treatment of Bowser before, but if theres one character that truly stands out for me it's Mario himself.
I don't know if I'm in the minority on this one, but I legitametly like Mario as a character and the M&L series is a great example of how to handle him:
He (and Luigi) has no dialog as usual, but he really doesn't need to in my opinion because the series handles expressions and reactions so well that the characters ooze with personality without even saying a single word.
It also helps that Mario is allowed to actually react to things, unlike the Paper Mario series were Mario is pretty static most of the time.
His interactions with Luigi are the best part thought, they may be brothers but Mario will get annoyed by Luigi's antics at times and sometime even forcefully drag him into a fight.
And then there are those moments were he stands up for Luigi like when confronting that Star door in Partners in Time, or making it clear to Starlow that it was Luigi who was ultimatly responsible for getting rid of the Shroops, those moments are just incredibly sweet and show a bond between the brothers that really should be more prominent in the enterity of the franchise.
This series also has my favorite piece of Mario artwork ever:
The sleeker look suits him really well and I like how they actually managed to make a more no nonsense kind of expression for Mario look naturel, especially compared to the Smash version: