Given the various edition shifts, I'd probably rank them like this:
Abjuration: Stronger in more recent editions, but in general you don't want to lose dispelling, banishment, and other various defenses.
Conjuration: Always a good school, gets stronger past 3.0 when it takes teleportation from Transmutation.
Divination: Not a powerful school, but has detections, scrying,
true seeing,
legend lore, etc, so you usually don't want to give it up.
Enchantment: Medium power. Spells like
sleep, charms,
domination, and so on are always useful, but this is a school that can be dropped without losing too much of importance.
Evocation: Strong in AD&D (particularly in 1e where
magic missiles and
fireballs and the like don't cap), but weak in 3.x and possibly later, where it's worth sacrificing.
Illusion: Depends heavily on DM adjudication, though 3e and later has stronger guidelines. As a result it can fluctuate between stong and weak. Illusionist has more unique and exclusive spells in 1e, and it's the only arcane caster option for gnomes in AD&D. Also a school that can be sacrificed without losing much.
Necromancy: Somewhat weak on the low levels, it's stronger later when the caster gains
death spell/circle of death,
finger of death, and
wail of the banshee. Arguably,
animate dead can be powerful too, but that depends on how much the DM wants the PC to roam around with a bunch of expendable zombie slaves. Not strong on direct damage though. It can be worth sacrificing, but a well-built necromancer has potential.
Transmutation: Later editions reigned it in from the ridiculous number of spells it had in 2e, but it still remains a powerful and versatile school, even after losing teleports to Conjuration. Don't sacrifice it.
Specialist picks:
2e: Go with an Enchanter, Necromancer, or Diviner if you want that extra spell. While Enchanter loses the big combat spells of Evocation, there's still stuff from Conjuration to attack with. You also sacrifice the instakills of Necromancy, but you've still got Illusions'
phantasmal killer and
weird to fall back on. Necromancer is good if you don't want to bother with illusions and enchantments. Talk your DM into letting you (ab)use the Complete Book of Necromancers.

(even if he says no, the book does have a pretty comprehensive list of the spells you can cast from several 2e sources). Diviner is better than it looks, since you're only dropping Conjuration and it's not as good as it will be post-AD&D. No matter what you pick, see if you can talk your DM into letting you use the optional abilities from Spells & Magic. Past core, elementalists, wild mages and some of the Player's Option specialties offer interesting choices.
3e: You get to choose oppositions. Avoid dropping Abjuration, Conjuration, Divination, or Transmutation; they have some important spells you don't want to lose.
5e: Haven't really played it enough, but Abjurer has some good defenses, and Conjurer and Transmuter look like they're pretty good too. Evoker really depends on how good the typical damage spells are in the game. Enchanter seems to have it's abilities based on Charisma, so it's a MAD wizard and I'd avoid it.