Do items drop based on class used or most used?

Solution 1:

The item drop system is a random process.

The class you are or the class you play most often have no effect on what item is dropped.

Pros:

Allows new and old players alike to earn unlockable weapons by investing playtime

Allows players to play as any class

Cons:

Many players receive unwanted duplicates

This issue was resolved by the introduction of the crafting and trading system

More information and source: Item drop system - Team Fortress Wiki