PFF has Suh as the top DT right now with a +27.6, Donald right behind him at 25.7. But... this is shaping up to be Suh's best year. He didn't get to a positive season per PFF until his third year when he was +15.2.
But these are not historic seasons for DTs (at least not yet). McCoy was +48.8 in 2013. Geno Atkins was +73.6 in 2012. Those were the best DT seasons since 2007, per PFF. The best rookie DT grade since 2007 was Pernell McPhee with a +20.7 in 2011, except he was really just a pass rush guy.
Geno Atkins is a great player to compare to Donald. Similar size, similar style. His rookie year in 2010, he was +15.6, followed by +27.2 his 2nd year, followed by the incredible +73.6.
The idea that Donald is making more of a splash than Atkins made his rookie year is really saying something... but it's saying something that every Rams fan already knew. Simply put, Donald is special. Remarkably special.