My favorite class is Alchemist. Tons of awesome options and flavor, and a solid level of general power / usefulness. Bombs, infusions, mutating into a hulking form, bottling oozes, building golems, using poisons, immortality, etc. The various alchemical items and dynamic alchemy, while not strictly tied to the class, help its flavor a lot too.
Second is Kineticist. It is the sort of thing I've wanted to have as a class option for ages. I love the elemental powers and having a blaster with the various utility powers. I think there's a lot of room for expansion on it too and see it going very nice places. (Notably, this is the one I've played the least on my list here, but I really like its general look.)
Third is Oracle. I like the flavor of the mysteries and revelations, and I enjoy playing casters. I tend to favor the spontaneous ones because their class-options strike me as generally more-interesting, and they're still full-casters and still quite potent.
Fourth is Witch. The hexes are my favorite part of 'em (even if there's a few mainstays that seem to make it into most builds).
Fifth is Sorcerer. I like it for reasons similar to the Oracle; I think it has less-interesting powers than the Oracle mysteries, but I still like the general transforming-into-an-inhuman-state aspects of a lot of the bloodlines.