It really depends on the map for me and I'm speaking strictly from a solo perspective.
Four devices doesn't give me much trouble on large maps, but on Glacier... OMFG... it's almost impossible to lure enemies away with how small the map is, and some of the locations have no cover whatsoever... horrible. Four devices on Jade is also horrible, especially because enemies can almost always shoot you from the balcony if the devices are near the middle.
Pizzas and Escort for the most part aren't bad, as you can use soft cover and lure enemies away if necessary, plus you can defend yourself (unlike four devices), but as already mentioned, if the map is Rio... just... ughhhhhhhhh! It is such a long path and there's not exactly a lot of cover to use, just awful.
Hacking I don't mind at all... except on London (eh, actually, when it comes to London I dread ALL objectives that aren't eliminate targets). The hacking on the LZ of Ghost though, that one is also really, really bad.
So overall I'm not sure I can pin-point one objective as the one that gives me the most trouble, as it varies too much depending on the map. Only thing I can say is that eliminate targets is certainly the easiest one.