Together with the symphonies, for me the string quartets as a whole are the most important body of work.
This is not to deny that there are a few pieces in the other genres, like among the late masses, the piano trios and sonatas etc. that are more important than certain quartets or symphonies. But I consider basically all quartets from and including op.9 major works of both Haydn and the string quartet genre and I would not say this about quite a few of the earlyish piano sonatas. As it is difficult and probably moot to talk about the relative quality, a case can be made that the quartets seem to have been more systematically written at certain points in Haydn's career and development than anything else (again, this is also true of a few pieces in other genres, e.g. the Paris symphonies).