All the good Evocation spells can be duplicated with (Greater) Shadow Evocation. Its offensive damage-dealing spells can be replaced with Conjurations and Transmutations, in most cases bypassing SR and just being better spells than what Evocation offers. The only must-have Evocation is Contingency, and you can save on casting time and the spell focus by using Greater Shadow Evocation anyway.

Most people will pick Enchantment as a second prohibited school, which is perfectly understandable considering how limited its spell selection is. The fact that nearly half the creatures in the game are outright immune to the entire school is another reason to just ignore it.

Necromancy is a common choice as well due to its utter lack of utility/buffing spells. Despite that, its offensive spells tend to be better than those of the Evocation and Enchantment schools, so it's sometimes a hard choice.

When specializing, you need to keep in mind that you'll have a spell slot of each level in your chosen school, so be sure to pick a school that has at least one spell of each level worth preparing at all character levels. Conjuration is the first choice of most people due to its versatility, plus you get low-level spells like Benign Transposition, Mage Armor, Glitterdust, Web, Greater Mage Armor, Bands of Steel, Dimension Door, Solid Fog, etc. that will be useful even at level 20. On top of that, you get variants like Abrupt Jaunt from PH2 and Enhanced Summoning and Rapid Summoning from UA/SRD. Transmutation is another good choice, you have low level utility spells like Nerveskitter, Rope Trick, and Haste, plus Spell Versatility will allow you to gain spells from prohibited schools.

Focused Specialist gives you almost as many spells/day as a Sorcerer, at the cost of versatility in both spells available and spells prepared. Three of your spell slots at each spell level are reserved for your chosen school, so carefully picking your school is even more important. With careful spell selection a focused specialist will always have a useful spell at the ready for any given situation, and doesn't really need to worry about conserving spells for later on since he has so many.