I highly recommend Mario and Luigi: Bowsers Inside Story and Dream Team. They are both excellent games with excellent stories and varied gameplay. I recommend playing bowsers inside story first since it properly introduces some characters but they are the best in the series in my opinion. Also if you want you should start with superstar saga and make your way through the series because a lot of people loved the first two games a lot!

play at least the first two Paper Marios.

TTYD is my favorite game of all-time.

Play 'em all. Especially the first two Paper Mario games. Just avoid Sticker Star, that's all you need to worry about.

All of them are great except Sticker Star

What did you not like about Partners in time?

Super Mario RPG is a classic. It's a little linear, though, so it can get boring.

Paper Mario is also a classic, and it has more freedom. It's also incredibly creative so it's great fun.

Paper Mario: The Thousand Year Door is one of the best games I've ever played, period. In my opinion, it's the pinnacle of the Paper Mario series.

Super Paper Mario highly disappointed me. It took away too much of the RPG elements and made it more action-focused. Didn't care much for it.

Paper Mario: Sticker Star brought it back to RPG, but in a weird and unique way that I can understand some not liking. I found it entertaining, though, but not as good as Paper Mario or TTYD.

I haven't touched the other Mario RPGs yet.