Excel is the big one. The web version is really, really good on its own terms, but desktop excel really is in a class of its own both in terms of capability and penetration.
Yes, it does give people regular reasons to point and laugh at it, and it's probably dangerous how much of the modern world depends on it, but you only have to use a competitor for a very short period of time to realise that displacing it would be a very, very tall order.
Except for compatibility (which will always be questionable with proprietary formats), I don't think most folks actually need Word or Excel. Most folks aren't delving into VBA and such, they just need a bunch of interlinked cells with a little math.
Compatibility is 90% of the problem, though. While it might be tempting to think that most people might not need Excel, the important thing to realise is that they think they do, the people who are sending them spreadsheets to look at think they do, and your beliefs aren't going to change that.
Yes, it does give people regular reasons to point and laugh at it, and it's probably dangerous how much of the modern world depends on it, but you only have to use a competitor for a very short period of time to realise that displacing it would be a very, very tall order.