Assuming we allow all official content—read: Dragon magazine—and accept any kind of fighting unarmed—andunarmed—read: not require that we stick solely tojust unarmed strike—and are talking about 20th level, monk wins hands down because of the variant in Dragon vol. 324 that gets wild shape. It’s not as good as a druid’s (fewer uses per day, slower progression to larger and/or elemental forms), but it is something we can actually value more monk levels for, which is more than we can say of fighter levels (or other, non-wild monk levels).
Wild shape may also be the only way these two classes have of accessing pounce while unarmed. Obviously the best approach is to just wild shape into something with pounce, but there are also some feat options for it that require wild shape: the Lion’s Pounce wild feat from Complete Divine lets you pounce by burning a use of wild shape, while the Bestial Charge tactical feat from Complete Champion lets you pounce on the round after using wild shape (or the same round, if you have the ability to wild shape fast enough). These are both very limited, but pounce is also very good. Still probably better off just wild shaping into a shape with pounce, though.
Wild shape is also by far the best way of grappling things yourself (as opposed to summoning grapplers, which is even better). Grappling is also something that’s fairly effective, ish, and its damage is based on your unarmed strike damage. So that’s nice.
Comparison
The comparison becomes difficult at this pointfighter probably can’t get pounce (Races of the Wild’s Catfolk Pounce or Player’s Handbook II’s Two-Weapon Pounce are conceivable, but both are mediocre, limited versions of pounce and very costly). However, the fighter can take Travel Devotion from Complete Champion, and can afford it more easily than a monk can—which is good, because wild shapethe fighter probably needs to take it 3 or 4 times. Pounce and Travel Devotion each have their pros and cons, but here Travel Devotion is better than these templatesa con primarily because the fighter would really like to charge, even when they achievebut without pounce probably can’t. Travel Devotion also requires swift actions, which is annoying because the same size—but not if wild shapefighter would like to have a swift action available to zap a doesn’twand of blockade achieve the same sizeto make sure they can dungeoncrash. AFlying arguably eliminates that concern, though.
Comparison
Ultimately, there is just so much that wild monkshape allows access to. Yes, the fighter can takeget the size by taking on Huge forms starting at 16th. A half-goristro is Huge at ECL 5thLA—and even manage it earlier than the monk can, allowing them to possibly be Huge before the wild monk haseven anyhas wild shape. But the LA also makes them preposterously vulnerable at low levels, so it’s not really viable until mid-levels anyway. At that point, wild shape is looking very competitive even if you don’t have Huge size yet. Once you get it at 16th, the monk is clearly superior.
For some numbers on the viability issue, an ECL 5th half-goristro also has, at best, something like 17 or 18 hp—any actual threats at this level are going to kill the fighter as an afterthought. The monk has at least 32 hp, and since they’re going for wild shape (and won’t need their own Strength or Dexterity after 6th), probably more like 37 or 42. The fighter gains more hp per level, though, and pulls ahead at 10th or so. So you’re looking at a sweet spot between roughly ECL 10th and 15th where the fighter is probably superiormaybe competitive, but above that or much below it, the monk looks far better.
But even in the sweet spot, the fighter is giving up a lot of their advantage because they have to take Travel Devotion multiple times, and they have fewer actual levels, and monk also gets some bonus feats, so the fighter might actually have fewer feats. Meanwhile, wild shape just... covers a multitude of sins. So even in the fighter’s sweet spot, the monk is probably better.