I liked 5 for solely killing Wario. Sky Survivors, Countdown Pound, Headwaiter, Heat Stroke, Merry Poppings, etc had all funny results when the computer loses. Otherwise, I don't play 5. I loved Super Duel Mode, though, and the introduction of Toad was nice.
I played the first 3 when I was very little, and couldn't remember, though the most memorable was End of the Line from Mario Party 3. Don't know why though, maybe it was funny to see the characters flying off the cliff, screaming when they took the wrong path. Though I did play Mario Party recently, I only played Tug-o-War, the river raft minigame, Piranha's Pursuit, Grab Bag, and some others because Wario screams "RUUUUUUUUHHH!!!!!" and "OH I MISS!"