You need a liquid instrument (s) with low spread erosion....ie if a day trader, what % is your spread (dealing cost) of the adr? Liquidity is generally related to volume, (although illiquidity can produce so-called 'erratic/exaggerated moves,) so choose an instrument (s) with good volume at the time of day you are trading it/them. Volatility is also good/necessary and is a product of the aforementioned. Add momentum too and you have yourself a ball game.
If you are trading multiple set-ups you may be better to stick with one/fewer instruments/time frames, but if you are only trading one/few set-ups then it may be better to trade multiple/more instruments/time frames.
The most liquid times of the trading day are generally when the major world trading centres are trading, and the major asset classes markets' are open and trading.
I choose to day trade gbpusd only across 2 triple t/f combinations for all the reasons I have set out above. I tend to trade an average of 4hrs a day and these hours are generally between 8am and 12noon London time, but this does sometimes vary, and sometimes I run over if there is a big data release in the U.s morning.
Intraday (mainly) swing scalper via a Supp/Res & PA based edge