Tyrian 2k and Jets'n'guns, they both have the biggest collection of weapons and upgrades afaik.
Tyrian 2k is free on gog.com.
Both have a similar concept.
You get coins for killing the enemy, which you can spend on buying weapons/upgrades.
Each weapon can be upgraded 3 times and up (some ten times!). Those are not regular increased fire rate or increased damage. You can fire more rockets/lasers at once. Shoots an additional plasma in a different direction etc.
The beauty is you can undo the upgrade and sell the weapon at anytime for the full price! You won't be bound to one single weapon every level due to this.
Note Jets'n'guns is inspired by Tyrian 2k