The easiest level to 100% is Ultimate Doom's E1M1. I've posted a video below showing you how to get it. This is NOT mine and all credit goes to BigMacDavis for this one. The video is on Ultra Violence level so if you're playing on a lower difficulty, you'll notice there are much fewer enemies (which is fine, just follow along and kill everyone you see irregardless of the fact that there are fewer enemies).
Along the way you'll also get this: