I suppose it depends if you are thinking of most important as a student or ones that are particularly useful through practice..
Anyway, I think Malcolm Payne's Modern Social Work Theory, is about as classic as they come in both instances! I personally found Assessment in Social Work (Milner and O'Byrne) so useful that I actually bought a second copy when I lost my first!
Other than that, the good old Mental Health Act Manual which is forever associated with social workers!
I'd also add Anti-discriminatory Practice by Neil Thompson in there. Lots of useful stuff although more recently I've read through Promoting Equality and think it has a bit more depth and would definitely recommend it.
I know that's five already - but a good dictionary of Social Work is handy as long as it's up to date