According to Google, I should use ga:
(Anata wa) sushi ga sukidesuka?
Why ga and not wa?
This is a big question, because it can't be answered fully without going deep into the semantics of は and が, and I linked in the comment to an excellent post that does just that.
However, without context, here's what we can say.
This is a neutral statement that simply means "I like sushi." In Japanese, the thing that is liked is marked as the subject. (If you've studied any Spanish, this will seem familiar.) When I say "neutral," I mean that you're expressing your like for sushi in a way that doesn't contrast with anything else. What kind of Japanese food do you like? I like sushi.
This statement feels contrastive. In English, imagine it as "Well, I like sushi, [but...]" with a lengthening of the last syllable of "sushi." On its own, it would sound very odd as a response to "What Japanese food do you like?" But it would feel at home in a statement like:
In general, if you're not using this type of construction in a contrastive way, choose が. This also goes for がきらい、が上手、が下手、and others.
I was taught that が is the subject marker which puts emphasis on the topic of the sentence. You can use either は or が in this sentence, but with が you're just emphasising how much you like something (in this case).