Well, if by "win," we're talking who comes out in a pure throw down, I'm kind of inclined to believe Discord is OP for a stand up fight.
The elements of harmony can petrify him, Tirek managed to neuter him in the Season 4 finale, and Chrysalis managed to keep him from using his powers with her anti-magic throne. However, when you don't have an element of harmony (Harmony being the opposite of Chaos), anti-magic artifacts, or a magic siphoning method around, then Discord's overabundance of chaos magic can pretty much defeat any opponent with a literal snap of the finger (and he might skip that if he's not feeling theatrical).
That said, if I were to lay out a rough estimate on who's going to win in a throwdown (through whatever means they have, with "win" defined as physical submission of the opponent), my theory is: Discord > Sombra > Havok > Hailstone.
Discord is top of the heap for reasons I mention above. Sombra is second place because he apparently had power enough to oppress an entire pony empire, a fairly mean trick for a magical race. Havok is third place because he's certainly got force but not necessarily oppress-an-entire-crystal-empire force. Hailstone's lore doesn't really establish he's proven his chops in a fight, though he's certainly magical.
However, I think it's barking up the wrong tree to assume Equestria runs on Dragonball Z rules. It's not really about power level. It would really seem to run on straight up best feels
rules. 'twas friendship that tamed the Discord. 'twas love that obliterated Sombra, expelled the changlings from Canterlot, and later reformed them. Twilight had the power of four alicorns aimed at Tirek, but in the end this proved insignificant to a harmony upgrade. Simply fighting never seems to resolve conflicts in Equestria.
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