Mentioned above MSCP is the smooth dive into slaughters, also Slaughterfest 2012 and SF3 (yes, skip 2011 and Smax, they are too much). Then you can try to play NoReason's Speedmaps 2/3 (or simply NoSp2/3), it is more like power fantasy, so it's about fun. Try to beat Holy Hell, old good classic, if you like monotonic grindy stuff, it's not hard, but requires some concentration.