well besides the extreme examples (snakes utilt and shuttle loop), what other attacks out there are so much better than kirbys fsmash such that we could classify them as opposites?
Id say... Giant Punch, ICs cg-usmash, ness bthrow, olimar usmash, wario uair.
I cant see how Kirbys fsmash, when compared to other KO moves in this game, could be called unreliable while others (that arent utilt) are, theres just not enough of a difference.
Tbh I don't even think Snakes utilt is good enough... what you need is many options that are "good".
If all a Snake will do is utilt, then it will suck, because your opponent can try to trick you to use it and punish (like just run up to you and shield, then punish). Even if the Snake is not obvious with his utilt, it's still really difficult to land if your opponent tries really hard to avoid it. (okay, maybe it isn't easy punishable, unless you're Diddy/Dedede)
The good thing for Snake is that he has many more moves that will KO too, even if they are slow you have to watch out for anything Snake does. And with his grab, which does 12 % I think, he can try to predict you and follow you near a ledge to trap you... nobody wants to get grabbed by Snake.
What makes him even more dangerous is his C4 (and sometimes his dsmash too). You need to watch out for so many things.
Ness' bthrow is really good, of course, but if it would be all Ness had, then it would suck, because your opponent will try to avoid it if he's in KO %. Luckily, Ness also has uair/bair, which can catch opponents that try to jump before you grab them...
I wouldn't say Diddys fsmash is reliable, but it is more reliable than Kirbys fsmash, imo, because it is way safer and a multihit (with the stronger hit being the 2nd one, while if someone spotdodges Kirbys fsmash he might only be hit with the weaker part, if at all).
It really seems to me like it's soo easy to shield Kirbys fsmash, and it's range also sucks... the lag makes it even worse, because you are able to punish. Think of Warios fsmash.
DKs dsmash would be really good, if it wouldn't have that much lag. If your opponent shields it, you'll be punished (badly?).
To have good/reliable KO potential you need many options and/or even setups. And of course they should be fast, safe and have good range.
A move on it's own would probably never be truly reliable... unless it's waayy broken.