Almost anything, to be honest. Matplotlib, R, Matlab, Mathematica etc. are all much nicer. Those will do most things ROOT does and be much less delicate. In a lot of places (especially outside CERN) Matplotlib is taking over where ROOT might have been used, but it's a slow process.

The problem is that ROOT still has a few very specialized features that its users still need and you can't get elsewhere. And there are a ton of legacy analysis tools built on top of it that are difficult to port because of how ROOT is. And a lot of its more extensive users are comfortable with it and have no motive to change (they're busy with being scientists).

I don't know anybody who actually likes ROOT, but it also won't be going away any time soon.