Georgetown in Guyana is below sea level at the Bourda Cricket Stadium, where a protective embankment encircles it. Also, there are quite a few coastal cities missing - if you are taking minimum elevations (which I assume you are, since you have hilly Monaco at altitude zero), then some on this list should also be zeroes, eg Dublin. Some aren't here at all, eg London (which is on the tidal part of the Thames River), Wellington and Buenos Aires.
Average height of Monaco > 0 by far : mountains in the back