With regards to the FTSE much depends on your proposed investment period....
If you said 2-years for example, then I would personally be very reluctant to invest in the UK given the issues around BREXIT, but if you said 20-years then that is an entirely different story and you need to take a view on whether you think the British economy will do well in the long run after it leaves the EU.
When it comes to the US, I personally am still upbeat about the S&P500 because it would appear that the US Government will be progressing with their stated policies to create economic activity within the US so should benefit US companies. Furthermore US Government strategies surrounding protectionism, trying to encourage US companies to relocate their operations back to the US from other locations and their interest rate policies need to be considered (there are a few more interest rate hikes expected in the next 12-months so there should not be any surprises, with corporates having factored these increases into their projections etc.).
The Eurozone is the one I am struggling with most, when I look into my crystal ball... at one level, corporate results are relatively good, interest rates are particularly low and the Quantitative Easing (though buying bonds) has put a lot more cash in the EU economies. However, that said, the German's obsession with inflation and concerns about Spanish and Italian debt levels cannot be ignored - both remain significant geographical and economic areas within the EU. Then you throw the likely exit of the UK from the EU into the mix and it's a toss of a coin almost with the most likely outcome being little change one way or the other over the next couple of years.
All of the above is just my own view, definitely not financial advice and sometimes sourced from a person who might be related to Mystic Megg