Some good wads I'd recommend playing:
Zone 400, really bland at first. The maps are small and get really challenging after a few, completely original music but sometimes the score is really unfitting.
Resurgence, from the creator of Speed of Doom wad, which means progressively longer and more difficult maps.
Abandon, takes the mantle as one of the most visually impressive and impossibly difficult wads. I barely managed to beat the first one without saves, but it seems nobody has beaten any of the other ones from start to finish.
Slaughterfest 2011 and especially 2012 have some quality maps in them, but a lot of garbage, especially the latter half. MAP17 by Ribbiks from SF2012 has to be the most memorable of them all. It's doable but will definitely challenge anyone.