Most of the tanks are the easiest mechanically, but imo require the most awareness and strategic decision-making. It would be overbearing if tanks were gated by mechanics, while also having to decide who to focus, who to peel for, when to initiate, how to counter-build (so they can stay alive long enough for the DPS to do their jobs), and remaining enough of a damage threat so they're not ignored by the enemy team.
All of those factors change, depending on team compositions, while the job of a DPS remains pretty much the same, relative to their type. Compared to tanks and supports, a DPS champion's lack of general control doesn't give them a lot of strategic options, which is made up for by having much higher damage output. Obviously, there are exceptions to this rule, which are typically the champions with the most balance issues.