S-Tier
Fireblast. Deals decent damage, and upgraded it becomes even better for crowd control! It helps survivability in 9 challenge and it's one of the only wands worth upgrading even for non-mages. It has a dangerous downside of burning scrolls and grass however, making it unusable in some scenarios, and the Battlemage's blazing hit can be detrimental.
Blast Wave. Movement options are always important to survive in the dungeon, and blast allows chasm kills and fishing! Not necessary to upgrade, and the Battlemage's elastic melee is redundant.
Corrosion. Deals good AOE damage, and unlike Fireblast it doesn't burn everything. Overall worth using, but the Elemental Blast effect deals terrible damage-over-time, without any extra effects or immediate damage. The Battlemage's caustic hit is unreliable as it doesn't last forever, and water cures it.
A-tier
Warding. Gives ranged damage and vision, good for darkness challenge. Upgraded it allows burst damage with wards but requires high costs in SoU, charges, and turns. The Elemental Blast is very very bad, it only heals nearby sentries and deals no damage or effect.
Lightning. Decent DPS that hits multiple targets especially in water, just don't get close. Battlemage hit effect is awful due to the staff's low melee damage, but it has great Elemental Blast effect as it stuns and deals full damage.
Displacing Dart. Helps a bit with crowd control on 9 challenge, but the fact that it can miss makes it too inconsistent.
Paralytic Dart. Same as above, but this has an awful duration. In a pinch it can help prevent ranged attacks, create some distance, but it's inconsistent.
B-tier
(IMO it's usually not worth upgrading wands past this point due to the high opportunity costs)
Prismatic Light. Good for sight with darkness challenge, and can rarely reveal hidden rooms. Prevents ranged attacks with blind. Damage is mediocre, and it's only passable against undead and demons. Battlemage's cripple-hit duration is way too short, it's extremely difficult to make distance using cripple at close range. Elemental Blast's search effect needs more range.
Disintegration. Good hallway damage against multiple enemies, it can destroy barricades but also grass. Battlemage melee effect is mediocre as it's just a little extra reach, using this staff to do kiting kills on 9 challenge can be difficult due to the low melee power.
Frost. Cooks meat and saves potions on fire rooms. Crowd control with chill is passable but not great. DPS is mediocre as it deals less damage to chill enemies, but it has a great Elemental Blast effect, dealing full damage + freezing.
Incendiary Dart. Decent source of damage that can burn rooms.
Blinding Dart. Prevents ranged attacks and allows surprise attacks to the target.
C-tier
Corruption. Requires too much SoUs + damage for reliable corruption, the opportunity costs are dangerous especially with forbidden runes challenge. And spending one charge + turn to give one random debuff is generally not worth it, as you need luck to find the right debuffs for the current situation. Weakened for example is not useful against mages, and Amok is not useful against single targets. I feel it needs to be a good debuffer in the first few hits before it corrupts? If the shot fails corruption it should give one debuff plus 20% chance for another one. Upgrading increase this chance by +5% each, capping at 50%.
Regrowth. You can get extra seeds, but it's better to farm with RoW as Regrowth requires too much SoU to be viable. Regrowth is especially useless in barren lands... it's a terrible challenge that removes all the fun stuff like dews and plants. The Elemental Blast and Rooted duration is too short for crowd control, and it doesn't deal damage. The Battlemage melee is good as it gives herbal healing on hit, but you unfortunately need to move and make distance on 9 challenge.
Living Earth. Only useful in the sewers. In general, minion builds in this game are very weak especially with challenges. The opportunity costs to upgrade and take care of your summons put players in a worse position. No-armor challenge destroys this wand completely. Elemental Blast is weak as it only deals half damage, and the effect only heals an already-summoned golem. It could instead either heal the golem or summon the golem if we don't have one.
Transfusion. Only useful for the charm to prevent ranged attacks. Sacrificing 5% health to heal allies is a horrible idea, more so with challenges! Almost every ally in the game have low HP and armor (spirit hawk, wraiths, elementals, earth guardian, wards, decoy, sad ghost without highly-upgraded armor). The only exception is prismatic image and shadow rogue as they use your armor, but the opportunity cost of spending one turn + 5% HP to give them measly heals is awful. We can use that turn for so much more, such as killing the offending enemy. No-armor challenge ruins minion builds even more, as it strips their already-frail defense...
Magic Missile. Low DPS but high ammo. Only useful to charge your other wands, and the effect is small.
Chilling Dart. 6 turns of chill (10 only on water) is not enough for crowd control. Frost wand deals more damage, doesn't miss, and is "free". Darts have a seed and gold cost, and the low-tier darts are too weak to be viable options. They need more bang for the buck!
Poison Dart. Poison damage is worse compared to incendiary, with many immunities.
D-tier
Healing Dart. Same reason as Transfusion; the opportunity cost to spend turns and resources to help allies is terrible as they die quickly. Sungrass seed in particular is valuable for "food" and safely dropping into chasm wells.
Cleansing Dart. Spending turns and seeds to clear ally debuffs is even worse! 10 turns status immunity for dumb allies is too small, and not worth using the seed as Mageroyal is also valuable for "food" with cleansing potion, and curing bleed on chasm wells. I've tried the dart forget effect on enemies but it's unreliable, and enemies never utilize buffs to remove. The only two mobs that use buffs is monk focused (which blocks an attack so cleansing dart is useless), and gnoll brute. I wonder if this works on gnoll? Though it'll still be useless as it's a dying buff from a single enemy in the game. Crystal Spire and Gnoll Geomancer's weapon immunity could be a buff instead, and if you remove it, you can attack them with regular weapons!
Adrenaline Dart. 5 turns of crippled is far worse than 6 turns of chilled, as the former only affects movement. It needs much more turns. Giving 10 turns of adrenaline is also too short for allies; most of them have poor DPS to make use of it. It's better to spend your turn whacking enemies manually. Not worth using the haste seed!
Holy Dart. The damage to undead is very low, and the blessed status duration is too short and unviable for frail allies. Starflower is another valuable seed for blandfruit and exp potion, and it's never worth crafting this dart.
Shocking Dart. It ignores armor, but the damage is extremely low. The electricity also doesn't do AOE like Lightning wand. Most enemies in the game have low armor anyway, the thrown weapons you find early on will deal more damage, and doesn't use up a seed.
Rotberry Dart. Low amount of uses, and extremely high opportunity cost, it doesn't justify giving up an upgraded wand from wandmaker, even if the wand is low-tier.
Yup, I agree it can be good! But I think its main problem is that it's only good in a vacuum, but when compared to other upgrade options or wands it's inferior. It needs tons of SoUs for reliable corruption, making it hard to use especially with forbidden runes, and spending one charge + turn to give one random debuff is generally not worth it, as you need luck to find the right debuffs for the current situation. Other wands like lightning can just simply kill things immediately instead of debuffing, personally I think wand of corruption needs to be a good debuffer in the first few hits before it corrupts?
As for my personal pick for worst wand it has to be transfusion, I mostly just use it to prevent eye beams, maybe some small shields. It has its uses but I prefer blastwave or fire!