Depends. Are you talking native free range (black wildebeest excluded) or a game ranch environment? A Nyala from behind a fence at a waterhole or during a drive about is a target. In their native free-range habitat in Mozambique a good bull is an extremely challenging game animal. Waterbuck will often seem to stand around on dry arid game farms, but can be very challenging in the savannas, marshes, and forests of their native range. Which lechwe? A red Lechwe taken after a couple of days wading around the Caprivi can be a spectacular achievement. The black, hunted in Zambia, can be the purpose of a single safari, and of course, the Kafue Flats Lechwe is a very specialized hunt. The red is stocked on some South African game farms, but I am unsure of the quality of such a hunt.
The Black Wildebeest is a creature of the game farm environment and can be very frustrating and challenging on a large property. If we are talking a game ranch, it has my vote, if free range then a 28"-30" nyala or mature red lechwe bull.