Group I metals are highly reactive and get more so as you increase atomic number, so if potassium counts as "low" that would be my bet... of course it would depend what the rules are.
What's low? Need it be pure? What tools do you get? Can you spend money?
For DIY, I'd guess helium.
Scandium?
Get it from a party balloon?
May be hard to keep for longer than a day or two though.
I'd say fluor because it's damn reactive (and thus, dangerous).