Ultimately, to save time I recommend: Marge vs. The Monorail and Lisa Goes Gaga. But if you ever decide to watch more:
For the best, and to gain an understanding of the characters, watch:
And Maggie Makes Three
Last Exit to Springfield
Marge vs. the Monorail
Kamp Krusty
For the best:
Homer's Enemy (only after watching a few episodes to understand exactly why what happens in this episode is funny)
Lemon of Troy
The Itchy and Scratchy and Poochie Show
You Only Move Twice
For the worst of the worst, watch:
Lisa Goes Gaga (the Lady Gaga episode, by far the worst episode of the series)
Bart vs. Itchy & Scratchy
Co-Dependent's Day
The Boys of Bummer (This is Homer's Enemy but done wrong)
and here's some of the most soulless episodes beyond the Lady Gaga shilling one:
The Musk Who Fell to Earth (Elon Musk shilling)
Every Man's Dream
The Drag Queen episode