I spent a few hours gathering and testing each and every throwing item in PMU. Here are my results!
I can sort each throwing item into one of 7 tiers, each tier having a different damage value. The higher the tier, the more damage the item deals, with the exception of tiers 6 and 7, which are special cases.
I’ll give each tier a power value, ranging from 35 to 75.
Tier 1
Power: 35
Tiny Thorn
Notes
This shouldn’t come as a surprise to anyone tbh
Tier 2
Power: 45
Stick
Cacnea Spike
Pencil
Coconut
Watermelon
Rare Fossil (!!!)
Notes
The Rare Fossil shocked me, given how rare of an item it is. Even more shocking is that it seems to be the only throwing item with a chance to miss, though I haven’t done enough testing to confirm that. The only upside it has against the others is that it’s not limited to a straight line.
Tier 3
Power: 60
Iron Thorn
Frozen Stick
Corsola Twig
Toxic Spike
Stun Spike
Icicle
Notes
I expected the Corsola Twig to be down with the Cacnea Spike, honestly.
Tier 4
Power: 70
Silver Spike
Notes
This is the first tier that isn’t buffed by the Adept Belt / Technician ability. So, technically, tier 3 is, in one sense, the strongest tier of throwing items.
Tier 5
Power: 75
Golden Fang
Sharp Bone
Notes
The Sharp Bone seemed to be very slightly stronger, but I’m pinning that down to RNG.
Tier 6
Power: 60?
Spirit Tag
Notes
The Spirit Tag is in an odd place where it seems its damage is calculated differently from other throwing items. My best guess is that it uses Special Attack stat instead of Attack, but I don’t want to go through the pain of grinding more for further testing 
Tier 7 (Fixed Damage)
Geo Pebble (15 Damage)
Gravelerock (20 Damage)
Blast Seed (40 Damage)
Notes
Yeah, I know blast seeds aren’t throwing items. But I wanted to include it anyway 