The Arnold Shortman Funko Pop from Hey Arnold was announced it would be vaulted about a year ago and since then the supply has really started to dry up. The demand for this classic Nickelodeon Pop stays pretty constant which will eventually make it hard to find.
The Gerald and Helga Funko Pops from the show are also vaulted. Gerald still shows available on the Funko app but it isn't available for retailers to reorder. All three have pretty good potential to increase in value over time.
Of course, this is speculation and not investment advise. Let me know if you agree or disagree in the comments below and feel free to mention any Pops you think are set to pop in price as well!