The ranseur has reach; that’s vastly more valuable than the slightly better damage of the heavy flail (and the slightly better criticals are barely a factor at all). Of the simple and martial weapons, it’s your best bet.
But as long as we’re discussing weapons with special properties for disarming, unarmed strikes have to be mentioned: they let you take the weapon instead of just removing it. Still not worth the −4 penalty you’ll be taking instead of the +6 you could get with a heavy flail or ranseur.
Beyond that, there are exotic weapons with some features worth taking a look at; a couple are even worth a feat:
The spiked chain, relative to the ranseur, lets you burn a feat threatening adjacent creatures as well as those at reach, and it’s got finesse. There’s also a drow scorpion chain which does slashing damage in Races of Eberron, if you have any need for slashing damage over piercing. And then Dragon vol. 319 has a “cahulaks” which is identical to a spiked chain except that it doesn’t have finesse, and it weighs 90% less, as if anyone cared.
We can improve on those, though, with the meteor hammer or rope dart from Dragon vol. 319, which are rather like a bludgeoning or piercing spiked chain with 15 foot continuous reach. They also count as monk weapons, if that matters to you. Seriously, these might be the only weapons in the game that might be too good for one feat.
Finally, the flindbar from Monster Manual III (part of the flind monster entry) has the lesser damage of the ranseur and the non-reach of the heavy flail, so you’re kind of spending a feat to get the worst of both worlds there. But what it does allow is a free disarm attempt on any threatened critical. And it’s got a native critical threat range of 19-20, which is really mediocre for anyone who wants to use critical hits for anything but obviously it could be a whole lot worse.
There are a bunch of other exotic two-handed weapons with the +2 disarm property, but none of them offer any notable improvements over the heavy flail or ranseur, so I won’t bother to list them. Most of them are double weapons, which you can’t even use if you want to keep your +4 bonus.
To take a totally different approach to this (and thanks to HeyICanChan for pointing it out), the quarterstaff of battle from Magic Item Compendium gives Improved Disarm for free. That’s far and away superior to anything offered above, simply because it saves you two feats (Combat Expertise and Improved Disarm).
I would recommend the quarterstaff of battle, really. If you have a ton of feats to play with some reason, and don’t mind burning 3-4 on this, which you really shouldn’t do, the meteor hammer or rope dart are very nice, particularly with Weapon Finesse. (Ask your DM if feycraft from Dungeon Master’s Guide II can be applied to allow finesse without the feat—as written, it only does that for light weapons, but maybe non-light finesse-able weapons should also get it.) The flindbar offers something unique, but it has no reach, and even with Improved Critical you’re looking at a 20% chance of getting that free disarm, at best, unless you start abusing 3.0 material. If disarming is your thing, you want to just do it, not attack and hope you get lucky on the roll.
I know your question opened with an acknowledgment that disarming is usually suboptimal, but I do want to caution you that really, it’s worse than “suboptimal,” it’s really bad. Even in a campaign where every foe is humanoid and relies on force of arms—which doesn’t sound like any campaign I’ve been in—a locked gauntlet matches the bonuses from your weapon and Improved Disarm for a measly 8 gp. And that’s assuming they’re only using the weapon in one hand, which they probably aren’t. This is why the quarterstaff of battle is so choice here: it allows you to do whatever disarm silliness you have planned cheaply.