First, ask your DM if you can qualify for abjurant champion with an exotic weapon; you don’t need proficiency in any martial weapons to take Exotic Weapon Proficiency, and there are some better options for exotic weapons.
If exotic weapons are not allowed: Longbow
There’s not a whole lot of point to getting any martial weapon; any situation where attacking with a weapon is a better option than spellcasting, the fight is either already won and you’re conserving resources, or else you would be better off getting out of there.
Therefore, you might as well take something you can trivially and safely plink at enemies with: the longbow is that. A crossbow of any stripe is awful without Rapid Reload, which you don’t want to spend a feat on. Either really wants Precise Shot, which is not an awful feat for a spellcaster, but still not really worth it. Particularly when it requires Point-blank Shot and you can just buy a rod of magical precision to cover your spells’ precision needs.
Attach a least crystal of return from Magic Item Compendium to it, because most of the time you’d probably rather have a wand or rod in hand; the few cases where you want it, you might as well be able to use it the turn you want it, and 300 gp is a paltry sum for Quick Draw.
If exotic weapons are allowed...
This gets more interesting. There are a number of exotic weapons with useful properties.
As general notes:
Good Exotic Weapons discussion
Haberdash the Masked – A build for a master of masks, which has an option, the gladiator mask, to gain proficiency with all weapons. Includes a thorough list of exotic weapons worth considering.
Dungeon Master’s Guide II has the Feycraft weapon template. For 1500 gp, a one-handed melee weapon can be made eligible for Weapon Finesse, or a light melee weapon can be made to use Dexterity even if you don’t actually have Weapon Finesse. Considering your stats, Feycraft should be applied to any melee weapon that qualifies.
- While you’re at the fey smith’s, consider picking up a feycraft mithral chain shirt and stuffing it full of thistledown padding: abjurant champion will make a +4 AC, −0 ACF, 0% ASF armor kind of redundant, but you can throw armor properties on it, which is supremely useful, and furthermore you can be caught without your luminous armor or have it dispelled.
A least crystal of return is still a good idea. I seriously tend to put one of these on every weapon or weapon-like item I ever buy, because free actions are so very nice.
...a braid blade gives free attacks (Dungeon vol. 120)
When you full-attack, and are wielding a braid blade, you get to make a free attack with it. This is a no-caveats, no-penalties, no-feats, stacks-with-everything extra attack à la haste or Rapid Shot. You’ll want something else for your other attacks, most likely; a quarterstaff or dagger is fine.
...a gnomish calculus lets you hurl alchemical weapons (Arms & Equipment Guide)
Also, it’s one of the most ridiculous (and ridiculously-cool) weapons ever printed, and has by far the best name of any weapon. The gnomish calculus allows you to hurl alchemical weapons like tanglefoot bags much farther than you otherwise could.
This is mostly pointless; alchemical weapons mostly stop being useful at like, level 3. But it’s still cool.
Gets massively better (but still mediocre) if your DM rules that, as ammunition for the calculus, your alchemical items get any weapon properties applied to the calculus. This is arguably RAW (the alchemical items are referred to as ammunition, and no exception to the usual rules for projectile weapons imparting their enhancements on their ammunition is made), but it’s an unusual enough situation that I would ask.
Finally, it’s described as a fancy sling. Gnomish or not, halflings are better with these than anyone else.
...a greatbow outdamages a longbow (Complete Warrior)
For the dead-simple upgrade to the martial suggestion.
...halfling skiprocks are allow you to attack twice as much (Races of the Wild)
If you hit someone with a skiprock, you get to make a free attack against anyone within 5 ft. of the first target at a −2 penalty. Requires tight enemy clustering, but can potentially double your number of attacks.
Plus, they count as ammunition, and therefore cost 1/50 to magically enhance compared to other weapons. So you can get dirt-cheap weapon properties, particularly stuff that don’t actually involve attacking like eager, warning, or defending. Honestly, you don’t need proficiency for this, but your DM might feel a little bit better about the cheesiness if you are proficient.
...harpoons let you apply some mundane battlefield control (Frostburn)
The guy you impale with this moves at half speed, and cannot run or charge. For a lot of enemies, that’s tantamount to saying they have no offense; you can do a lot worse with a weapon. They can remove it as a full-round action – but it deals its damage again, and they just wasted their turn. If an enemy actually does that, you should thank them for it.
Also conveniently a thrown weapon, which is takes advantage of your being a halfling.
...a long staff can make you impossible to flank (Complete Adventurer)
You have to use the Total Defense option, or take Combat Expertise, but still, you get the option of eliminating flanking. If you’re hurting, just need to survive a round, and have a pair of rogues on you, this is effectively a huge boost to AC plus totally wrecking their damage output. It’s niche as hell, but man is it awesome when it does work.
Then again, you should almost-definitely be getting heavy fortitude on that feycraft armor or on a mithral buckler sooner rather than later. (Soulfire from Book of Exalted Deeds is your other priority, if you were wondering; having a +1 soulfire feycraft mithral chain shirt and a +1 heavy fortitude mithral buckler is my usual goal for armor.)
...a rope dart (meteor hammer) has the best reach (Dragon vol. 319)
This thing is ridiculous: 15-ft. continuous reach, à la the whip, but none of that nonsense about not threatening or failing against armor. Two-handed, but it can be finessed by default; ask your DM if you can apply feycraft to it and if so, whether it works as it does on light weapons. RAW, neither is true.
There are actually two weapons presented in Dragon vol. 319: the rope dart, which is described in detail, and then the meteor hammer, which is exactly like a rope dart except that it deals bludgeoning damage. Proficiency in one counts for the other.
...or a whip-dagger (Arms & Equipment Guide, Dragon vol. 353, Dungeon vol. 134)
If you don’t get to apply feycraft to the meteor hammer, you can ask after the whip-dagger instead: same reach, one-handed-but-finesseable weapon instead. No threatening with that one, but at least you can attack armored foes with it.
...or kusari-gama (Dungeon Master’s Guide)
If that doesn’t fly, the kusari-gama from the Dungeon Master’s Guide is a sure thing: it’s a light weapon that is otherwise a lower-damage spiked chain (in complete defiance of reality, I should add). Less reach on it, though.
...a razor net annoys enemies (Dragon Compendium)
This is basically a net that deals 1d6 damage, cuz why not. Nets can pretty much shut someone down; that’s highly useful.
Feel free to grab the regular net if Dragon Compendium isn’t available; the 1d6 damage is minuscule anyway.
Actually, consider grabbing (razor) nets even if you don’t take proficiency in them. They’re a touch attack anyway; even at −4 you can reliably hit that on some enemies. Plus, thrown weapon, so you’re already starting at +1.
...a Xen’drik boomerang qualifies for Boomerang Daze (Races of Eberron)
I know, it’s another feat. It’s also absolutely ridiculous. You get to push a hard Fortitude save versus daze on every single attack. This is so good that it will be better than casting a spell fairly often. Nothing else on this list is even close to claiming that. Daze is a status condition that very-nearly-no-one is immune to. Favor of the martyr is a 4th-level paladin spell that provides immunity to daze for 1 round/level – and it is just about the only thing that does that, so shenanigans to get it onto non-paladin spell lists are common. Anyone who hasn’t engaged in shenanigans (and isn’t a high-level paladin) is otherwise vulnerable to this.
If you want double-down on this, and spend another feat, Boomerang Ricochet can allow you to daze two people per attack.
Also, it’s thrown, so your halfling bonus applies.