I always preferred Devils back-in-the-day and that preference has continued over the years. Devils have a sense of menace behind them because of the organization and sense of purpose that they bring to the encounter.
There is also a sense from the players that Devils are not "sort of" evil. They are, literally, evil incarnate. There is nothing worse -- except an Archdevil. And all of them play for keeps.
I think it is also fair to say that as a DM, I never feel that I am crossing the line when playing a devil in an encounter in a manner which optomizes its attacks or causes it to do something ... devilishly clever.
For most encounters (most, not all) that are played during a typical game session, there is a certain social contract that the DM is not expected to go *all out* against the players without execeptionally good reason. Whether this "gentlemen's agreement" is to "simulate" the reduced intelligence of the foe or for some other contrived reason is arguable. But whatever the source of the understanding, it's generally expected by players that there is a line that is seldom crossed. I don't mean to say that the default play method is "hold back." But there is a difference, in my mind, between playing the foe reasonably -- and playing him as diabolically nasty as one can. Because once crossed, to go all out against the players can transform the game session into a game of DM vs. the Players.
Usually this is unhealthy. However, when the context allows it and everyone involved understands that the gloves are absolutely off and every possible advantage will be pressed, the game takes on a dangerous edge.
Some encounters, by their very nature, demand this sort of "gloves are off" approach. For me and the people I play with, Devils are the foes where I have always felt that the "social contract" in the game REQUIRES that the DM not hold back.
Everybody at the table understands and EXPECTS that the devil(s) will use every resource that it has to prevail -- as cunningly as possible, too.
As a consequence, my players have always understood that devils are incredibly dangerous and will be played for keeps. The tension level always rises as a result. There is a palpable ... respect ... that descends during any combat when there is, literally, devilry afoot.
