There is no easy answer. It depends on where your edge is. You must have an edge. Without an edge all markets are hard. But i've traded at professional firms for about 10 years so some observations..
If you are day trading, then spot FX is a losers game, the spread will kill you. Ive worked with hundreds of professional day traders at not one cash FX trader was profitable. Paying 1-2 ticks spread on trade will kill you unless you are going for at least 30+ tick winners.
If you are new to trading start off with the slower markets so bonds (nothing longer in duration than a 10yr). These markets arent "easier" to trade, but will give you more time to figure out what you are doing before losing your account.
Once you develop an edge in one market (which takes a long time) then look to see if it works in other markets and you will start to find markets that suit your personality. Some people like fast thin markets, others like slow and liquid... there is no one market better than the other.
As a very loose rule of thumb in my experience the less saturated/heavily traded markets have easier edges as there is less competition. But some people I know started off in a saturated market and did fine.