The primary advantage wizards have is their versatility. Compared to a Sorcerer, a Wizard has fewer spells per day, but has a much wider selection, and gains new spell levels sooner (Levels 3 5 7 9 11 13 15 17) than a Sorcerer (4 6 8 10 12 etc). A wizard can learn every spell in the game, while a Sorcerer has a much smaller selection.
Which is the 'most powerful' really ends up depending on the module. In a module where magic items capable of casting spells as well as scrolls are plentiful, then the Sorcerer and Warlock can potentially edge ahead. Especially if items with immunities are plentiful.
However, Wizards also excel at multiclassing, there are several PrCs that boost the Wizard's power drastically, the primary ones being the Archmage and Red Wizard of Thay. The Red Wizard in particular can potentially boast spell DCs in the neighborhood of 50-60ish, and this carries over to Epic spells of the same school. A Necromaner Red Wizard, for instance, can cast Momento Mori or Tolodine's Killing Wind with very high DCs. The Archmage class plays more towards the Wizard's versatility and allows one to alter the energy type in a spell (fireball that does cold damage instead, lightning bolt that does acid damage instead, ect) and can ignore the dangers of full-PvP setting servers (spell shaping = can cast AoE without harming friendlies).
I'm sure other folks can think of other combinations. Oh, and you can't forget the Lich either...the Demilich in particular is, basically, neigh-invulnerable and still has most of the power of a high level Wizard. i.e. overpowered (though that's intended)
Though at lower levels, the warlock and sorcerer do hold an advantadge, especially in the single digit levels, where a wizard's spell arsenal is tiny and expended quickly.