While other Heroes have access to stacking effects, Pygmalien has the most.
This works in tandem with his economic strength since buying and selling items are often the triggers for his stacks.
For example, Beast of Burden gains more Damage every time you buy an item.
Similarly, you have the Lemonade Stand which gains Max Health every time you sell a Small item.
Be sure to prioritize upgrading these items as soon as possible, as their stacking amounts increase based on their rarity tier.
If you anticipate a potential upgrade (via nearby level-up reward or upcoming shops), consider holding onto any applicable items before selling them!
Example Items
Dog
Dog, just like Lemonade Stand, scales whenever you sell a Small item. This is great since Small items are cheap, and even better when you can take advantage of events that produce multiple items such as Gumballs or Chocolate Bars.
It's a great choice early on since it doesn't require a ton of gold to build up.
Pyg's Gym
Pyg's Gym is a little more specific in that it requires you to buy Weapons in order to scale. However, it shares the benefit of Lemonade Stand in that it increases your Max Health regardless of whether it stays on your board. As an added bonus, it grows in value every time you purchase a Weapon.
While it can't benefit from the likes of Chocolate Bars, it does remove the size restriction, making it actually easier to scale in some cases. It's definitely worth the investment if your Income can afford it.
Pawn Shop
Pawn Shop has the most flexibility of the bunch, allowing every sold item to scale the Property. However, it does take a bit of time to get going and doesn't provide any actual benefits (beyond gold investment) while just sitting in your Stash.
But once it's had enough time to scale, you can either take advantage of its huge Max Health buff by including it on your board, or cash it in for a ton of gold.