There are dozens of threads in the forum addressing this specific question (which is, of course, a good one).
By far the commonest answer and advice is EUR/USD. That doesn’t necessarily make it the “right answer” but it’s certainly what most successful traders would advise.
It’s the pair that has by far the highest liquidity, smallest spreads and dealing costs and it’s normally “fairly well behaved”.
That’s normal: financial instruments will usually trend in different directions on different timeframes.
Some people (especially trend-followers) trade only when the trend direction is the same on the timeframe they use and one or two higher ones.
For the most part, which timeframe to work from depends on the style of trading you want to use, and your own hours of availability. There’s no “one size fits all” answer.
The post linked to below (click the green link) is the most helpful one I’ve seen, on this subject. Have a good read.