The MP system was best in KH1. I actually found the system in KH2 more frustrating and harder to utilize than KH1's, since the moment you Cure, boom, magic is gone. I won't go further on that subject though, since everyone has more or less touched on what made the KH1 magic that much better and useful.
That being said, KH2 had a much better version of Fire (Firaga especially), Reflect was pretty useful in the FM fights, and the Magnet line of spells from the series are wonderful, especially if mixed with Mines or Balloons.
Ideally, I'd love for KH3 to have the MP system from KH1, but give us a wide range of physical and elemental magic we can equip and choose to use from KH2, BbS, Re:coded, and DDD. Zero Gravity was much better than Gravity, if you ask me, the Surges saved my life SO MANY TIMES, etc. Meanwhile, certain abilities should be Limits again instead of just abilities (though some being shotlocks is also fine).