In terms of newspapers I do the Listener, Inquisitor (Saturday Independent) and EV (Sunday Telegraph) every week. In general, the Listener is toughest (this week particularly brutal), then Inquisitor, and EV the easiest, though it does vary a fair amount. Very roughly, on average, the Listener takes me twice as long as EV, and a bit less than twice as long as the Inquisitor. Maybe one in 20 EVs is on a par with the Listener in difficulty terms, but that usually indicates an easy Listener, rather than a hard EV.
The Spectator is of similar standard to an EV, and specialises in puzzles with some unclued answers that need to be filled in, all linked by a theme.
The Magpie magazine publishes six puzzles monthly, graded A to E with one a numerical. I would say a Listener would usually be a B or C, so you can imagine the D and very rare E grade puzzles are about the most difficult puzzles in circulation.
Of course it's all subjective, but that's my two pennorth . . .