Is Monaco the hardest formula one track?

The most difficult one is the Circuit de Spa-Francorchamps, the Eau rouge curve. Nobody can go wrong.

Imola old when senna and ratzenberger died was the most difficult very irregular and fast.

The Monaco circuit is the hardest one this year but Nurburgring Nordschleife is tougher when the German Grand Prix is held there.

Hardest in what way?   For me apart from the tradition surrounding the race, the venue, it's the most boring, and potentially the most dangerous.   Hate it.   But then I'm not into street tracks which tend to be a procession, altered only by pit stops.

Formula 1 track?

yeah easily, because it's less about power and more about what the driver is capable of....it also demands the most concentration and car control..and i agree with @Vermulen, it is the most boring in many ways for pure race fans

Not for pro drivers, they tend to find tracks like spa and Suzuka more challenging due to the high speed corners

By far. 

It's streets are narrow, and overtaking, is virtually impossible , because of all the tight corners .

Basically the race is won or lost on the qualifying laps.   If you start at the head of the grid,  and enter the first turn ,  first...... you really have to screw up or crash, to lose the race. 

Most F-1 races are on a track, made for F-1 races....... long straightaways, with ample chances for passing and strategy. 

Monaco, is run on basic city streets.....which does not really have any sort of layout for racing. 

Car failures  or crashing  are the most likely cause for losing if you start at the head of the pack . Anyone starting 5 or more cars back, has virtually zero chance of winning. ----and that's probably generous .

More like first 3 really.