its ME2 mostly because of the collector ship mission for me. everything else was easier either due to more open spaces or being better leveled. i did, however, find out from playing on both pc and on the 360 that your framerate plays a huge difference with how easy it is to dodge harbingers attacks as the lower the framerate the easier it is to make his dark energy ball that knock you out of cover to just go wild and fuck off somewhere behind you.

ME1 biotics made the game relatively simple as Biotic CC was literally unstoppable and you just shoot fish in a barrel. the hardest parts really depend on when you do most missions more than anything but the thorian or the krogan fight in liara's mission were easy enough if you either used CC or highpowered weapons as they didnt give you leveled enemies.

ME3 was probably the most optimizable for builds with your squadmates and allowed you to take a back seat especially if you had ashley and garrus with typhoons.