Bowser's Inside Story is my favourite entry in the series. It's not perfect though, as its quite verbose, volume of mini-games can be a bit much at times, and I'm not a huge fan of the sidescrolling worlds. Everything else is ace though. The world and characterisation had me hooked. Playing as Bowser finally brought us the fleshed out perspective they've been dabbling with in Super Mario RPG and Paper Mario. The dynamic between Bowser and his crew was the most endearing part of the game.
Superstar Saga is the other one I consider a top tier game. It's a lot more "vanilla" than Bowser's Inside Story, and I don't like the Bean people, but it's a game that felt clever and complete. It's probably still the best one to play first in the series, but since you've played it, you've got that covered.
The other two are a bit iffier. Dream Team is a competent game, but it magnified all of Inside Story's problems. A wordy game became wordier. It keeps trying to freshen up the gameplay with new mechanics and mini-games and attacks, causing the tutorials to keep coming from start to finish. It also brought back the sidescrolling that I didn't care for. I still beat it, and liked most of it, but I looked up against picking it back up every day.
Partners in Time I never really cared for. A lot of the problems of the other ones are present, but also increased the complexity of the combat. I'm sure people more skilled than me cough manage the 4-button combat, but it was too much for me. Not being able to master it, really put me at a disadvantage in the combat, causing battles to take way too long. A boss battle could take up to an hour at the rate I was going. Also Baby Mario and Baby Luigi were there, and I generally do not like these characters and their sound effects.