Wizard is easily the best. Master Specialist Abjurer specifically. You can even go straight into Initiate of the Sevenfold Veil for some really nice defense abilities and the absolutely brutal Prismatic Doom. Here's what I'd do:

Illumian with Krau-sigil (and Naun or something; anything of any use)

Cloistered Cleric 1: Pick Knowledge Devotion, Inquisition-domain and something like Competition (for a small improvement on counterspelling), Pride (for superb saves) or Undeath (for extra turnings).
Abjurer 1 (banning Evocation and Enchantment/Necromancy depending on your focus)
Abjurer 2
Abjurer 3
Master Specialist 1-6 [CMage]
Initiate of the Sevenfold Veil 1-7 [CArc]
Master Specialist 7-9 or Archmage 1-3 (Archmage gets Mastery of Counterspelling, Mastery of Shaping and Spell Power, for example)

With the following feats:
Divine Defiance [Fiendish Codex II] - key to making this work
Elven Spell Lore [PHBII]
Arcane Mastery [CArc]
Chain Spell [CArc] (Chained Dispel Magic is offensively very powerful)

and usual metamagic.

Basically, you use Divine Defiance (hence the Cleric-level) to get free action counterspells, Inquisition-domain for massive buffs to dispels and Master Specialist for some extra buffs to it. Further, Elven Spell Lore and Arcane Mastery mean you'll never fail the checks and Initiate just synergises well with all the Abjuration. Thanks to Illumian's Krau-sigil, your Cleric Caster Level is 3 and thus you qualify for Divine Defiance without Practiced Spellcaster: Cleric. Further, the same Sigil keeps your Wizard Caster Level maxed even with the Cleric-level.

Now just use Battlemagic Perception [Heroes of Battle] & Ring of Spell-Battle [Magic Item Compendium] to detect spellcasting and throw it in their faces. Add Reliquary Holy Symbol, et co. and you'll have enough turnings to always fuel Divine Defiance. Prepare a variety of dispel magic-effects (to get past any possible Rings of Counterspells your opponent has) and you'll be golden. There's a variety of them in Spell Compendium.


Read this treatise on countermagic.