another vote for bushbuck
From my experence;
I think that their sheer numbers eliminates the kudu. Big ones are certainly harder to come by but kudu can be found in very good numbers in most of SA.
Eland are big and easier to spot in some terrain. We had very good luck finding and following tracks in the heavy veldt, and when the wind cooperated getting in for a shot.
Nyala territory is much smaller than kudu but finding good numbers in Zululand had good success glassing the very hilly terrain.
Bushbuck; I know of many folks that have taken good ones, some with luck, some with perseverance, and some thru access to very fertile areas, and some by shooting after dark when they are most active. To date in spite of good ph’s, decent areas, and several days on several safaris, Bushbuck, have eluded me, at least one of shootable standards.