I thought SPM was the best in the series. For me, the continuum goes:
SPM > PM >>>> PM2:TTYD.
SPM nailed the "platformer with RPG elements" shindig, and the "flipping" mechanic was excellent, if a bit overused. If anything, the game's biggest flaw is that it kept the wrong RPG elements from the first two PM games - they should have done away completely with inventory items (which were clunky and unnecessary to use in the real-time) and cut down a lot on dialog, while maintaining character customization in the form of badges or something else.
"'Casual games' are something the 'Game Industry' invented to explain away the Wii success instead of actually listening or looking at what Nintendo did. There is no 'casual strategy' from Nintendo. 'Accessible strategy', yes, but ‘casual gamers’ is just the 'Game Industry''s polite way of saying what they feel: 'retarded gamers'."
-Sean Malstrom
