Oops, then it''s getting a hard one....
It must be LCD, it wasn''t the Mr. DO, so I think we must limit to the really tiny ones.
Options that are arcade classics in handheld:
Bandai - Zaxxon, but this one is easy to get in Europe, great game.
Bandai - Burgertime (LCD), was not sold in the US, so would be weird.
Mattel - Burgertime: essentially the same game, but then the US version. I have it: very cool
Bandai - Pengo: only a half-baked arcade conversion. The VFD Pengo is real Pengo, the LCD one is a fun game based on Pengo. Not classic enough.
Halion - Q*Bert: classic game, but very unknown and not so classic company, even though it shares the common Bandai handheld with ministick shape.
Bandai - Frisky Tom: nice classic conversion, but not very classic in the sense that it is well known as classic....
One of countless Pac-Man versions
One of many Super Cobra versions: not likely, they were common in the EU
Best guess: Mattel Burger Time