How does one define most active? :)
Longest continuous eruptions.
Interesting, I thought that was Stromboli. (Aristoteles, Strabo and Plinius already mentioned Stromboli - "Strongyle" - in way that lets us assume that it active since at least 2500 (!) years). This difference is probably due to what one considers "active". Probably another one of these impossible to clearly define and set records.
Stromboli in addition was growing under the sea for perhaps some considerable time before that. Sangay also often gets credit for being the world's most 'active' based on regular 'explosions'. Is this 'continuous'? Is Arenal continuous? Where does one 'eruption' start and another finish?. Further, Kilauea has been in a state of eruption since 1983, which admittedly is more recent than 1968, but has had a permenantly growing lava field. Iceland, while maybe considered more a volcanic region than a individual volcano, has been growing for quite a while too owing to the ridge location. In other cases, does a lava lake imply continuously active even? Or fumerolic activity? Ah, volcanic shemantics....
Interestingly, the book I just got on Ecuador says that Sangay is the most active volcano in the world. It seems the world has many "most active volcanoes". Yet another one to chew on.