(Posting from my phone, I'll fix any wonky formatting when I get home. Sorry for any inconvenience.)
If you're just looking to optimize purely for big single-shot damage for huge crits, you're going to want to get your weapon dice as big as possible.
Get the Strongarm Bracers (MIC) to let you wield a weapon one size bigger than normal to move your longbow from 1d8 to 2d6. Then go ahead and eat the -2 penalty to go another size up for 3d6. -2 for, on average, +3.5 more damage is almost as efficient as Power Attack with a two-hander, so that feels like a fair trade. More weapon sizes boosts get even more efficient from there.
This may be one case where including Pathfinder material could help you out. Namely, a spell and a feat chain.
The spell is called gravity bow. It's a 1st level ranger spell that increases the damage of your bow by one size category for 1 minute. It's great for putting in a wand because nothing about it cares about caster level; get a wand compartment (Dungeonscape) put in your bow so you always have it ready. If you're using hunter's merc from a wand as well (wise decision), get a wand compartment for your armor spikes, too. The spikes are also great for threatening adjacent squares.
The feat chain is the Vital Strike line. Vital Strike is a standard action to add your weapon dice as bonus damage. The first requires +6 BAB, Improved (+weapon dice x2) at +11, and Greater (+weapon dice x3) at +12. As bonus dice, these don't get multiplied on a crit but they can be substantial with big enough weapons.
Beyond size boosts, look for static +X bonuses that will get multiplied on a crit. Str bonus composite bows are the place to start. If you're expecting your Str bonus to vary a lot, think about working in a bow of the moon, a relic weapon of Corellon Larethion (MIC). It auto-adjusts to your Str. Knowledge Devotion (CC) is also a decent source.