I use fsmash mainly when I think my opponent is going to approach me. The fact that it's partially disjointed really helps with this and not getting punished. Although usually I only use it when the foe is at low or mid damage (although to be fair, even at mid damage it can kill, so w/e). People have a tendency to play more careful when they have high damage, and it's just unnecessary at that point when you have the ftilt available.
Anyway, I use the dair, ftilt, uair, bair, fair and dash attack to kill. Dash attack probably the least of any of the above, and the uair/bair/fair usually in edgeguard, while dair kills tend to be off the top for me more often than the bottom (although I still do get quite a few spikes). Occasionally I'll use downB (aerial non-spike version) to kill, or ganoncide if I'm at a high percent. Really, it's hard to definitively say I kill with "x" the most, as I really just use whatever I can hit with at the time, and they usually die. lol