We're trying to fit a nice notifications tab icon for our app. However, we couldn't find a global icon to define notifications. Facebook uses a globe (we can't use that, as we use globe to "discover/explore" tab in our app). Instagram uses a heart inside a chat bubble, but I don't think it represents "notifications" globally, it's more of a "like notifications". Twitter uses a bell, which, we think, is very ugly and doesn't fit the look and feel of our app. We've also seen exclamation marks inside triangles, but they represent more of a warning/error instead of notifications. We're looking for a good candidate for notifications icon. Is there a good one that everyone uses, or does it have the same fate with "share" icon in UX community?
In my opinion it depends on what these notifications are.
If it's about a social network, an inbox (envelope) or chat bubble icon makes sense, as most of the notifications will be about other users messaging them.
On the other hand, if it's a service provider where most notifications will be automated processes telling you that an update finished or your server burnt down and your data is lost, it may be better to use a "I" (information) icon, or a warning triangle.