For a variety of reasons, Vanguard is probably one of the best introductions to Doom modding there is. For one thing, it's a very well-paced episode replacement, not short enough to beat in one sitting, but not long enough to feel like it's drags. For another, it's authored by skillsaw, who's undoubtedly one of the most talented and well-rounded mappers in classic Doom. Vanguard is also his only major work without any custom monsters whatsoever (others being Lunatic, Valiant, and Ancient Aliens, which all have at least two custom monsters each). But perhaps most importantly is the scope of gameplay variance captured in this WAD. There's bite-sized maps, adventure maps, arena maps, slaughter-lite maps, you name it, all of which are anchored by a very reasonable difficulty curve. While Vanguard seems unassuming on the surface, it's a classic through and through, and highly recommended for anyone wanting to dip their toes in the wonderful world of Doom modding.