I think Ganondorf, Mario, DK (Fair), and K. Rool (Bair) have the meatiest spikes. Ganondorf has the electric freeze-frames to give it that extra punch, and the others just have the perfect "dunk" animations. Honorable mention to Dedede Dair, which has an animation that looks like it should be meaty, but the blunt hammer hit sound effect kind of neuters it.
Landing Pit's and Ike's spikes is satisfying in that it's inordinately difficult to actually land those things with their small, precise spike sweetspots. (Conversely, spikes that are too easy to land like Ivysaur or Kirby Dair are proportionately less hype.)
And of course, not much beats landing a suicidal stall and fall Dair spike offstage.