No idea why you picked Dispater. He is not even that memorable among the Arch Devils. Asmodeus the actual King of Hell would be a better choice.
I would also take out Venger, Makessa and Drelnza, and put in more iconic villains like, Tiamat, Orcus and Demogorgon.
...because the original question asked to pick 12 villains to serve as "foils" for the chosen heroes (and possibly the anti-heroes). Picking Asmodeus, the archlord of the Nine Hells... and Tiamat... and Orcus... and Demogorgon... to face off against Minsc and Tanis is a bit ridiculous, yeah?
On my list of 12, Dispater is the only "power" (i.e. god-like entity) included. I also specifically called him out as masquerading as someone else; concealing the bulk of his powers. Considering he's the ruler of an entire planar layer, with potentially millions of devils at his beck and call, it's arguably ridiculous to even include him in the list. But still... what the heck.
As to his prominence, I'd have to dispute you on that one. Dispater is arguably the most significant of all the archdevils; perhaps all the lower planar powers.
1) He was one of only 8 archfiends listed in the original 1e Monster Manual.
2) He was the only one of those eight archfiends to also feature prominently in 2e. Geryon lost his layer, Baalzebul was replaced by "Triel the Fallen", Asmodeus became "the nameless lord of Nessus" until the very last days of 2e, Orcus "died" and became Tenebrous, Demogorgon and Yeenoghu were relegated to minor status in the Monstrous Mythology splatbook, and Juiblex nearly vanished. Dispater, by contrast, was a major Planescape player and the central character in the module Fires of Dis.
3) He was back in 3e, along with all the other archfiends... and he was one of only two archdevils to get an official D&D mini (the other being Mephistopheles).
4) ...and, surprise surprise, he was the only archdevil officially statted up in 4e (Manual of Planes)
5) ...and he's still kicking along in 5e.
He is, other than Asmodeus, the only archdevil who has gone the entire distance. Tiamat was replaced by Bel was replaced by Zariel. Mammon was renamed Minauros for a time. Belial was supplanted by his daughter Fierna. Geryon was replaced by Levistus. Moloch was replaced by the hag countess Malagard was replaced by Glasya. Baalzebul was renamed Triel for a time. Mephistopheles was betrayed and replaced by Molikroth (2e), then it was revealed that Molikroth was Mephistopheles all along (3e ret-con).
So, don't sell the Big D short. He's a survivor.