That's a pretty hard question, since there are sooo many great games for the GBA...
I'd say, any of the castlevania games to start -- not much story, but they have a great smooth difficulty curve and are just completely fun to play (indeed, addictive).
[Luckily with an SP, you won't be bothered by the rather dark color color palette used on the first one, which annoyed many people with original GBAs.]