If you come to Budapest you need to get a metro pass for the duratiion of your stay.
Cheap and convenient.
Budapest now has a digital metro pass which I started using earlier this year.
TexasTravelMom was good to point out that you have to take your phone, go to the app, scan the VR code prior to boarding.
I sort of missed this but never got caught. Some cities require you to scan something to get through the turnstiles and some even require you to scan when you leave. Budapest on the other hand has no turnstiles and enforces by random checks. I get checked maybe 1 in 50 rides.
Given that, I quit the electronic version and bought a paper card. I don't have to scan anything, just produce it if asked. To be sure I have it I put it in a plastic sleeve on my keychain. So much easier.