As for kudu or nyala that is a hard choice as they are both beautiful animals. Kudu is normally a harder hunt and gives you a run for your money to get one.
Waterbuck was one of my favorite animals to hunt as was eland. Good luck and enjoy the trip no matter what you pick in the end
I'll second @billc here. I went into my first safari wanting Kudu and Nyala and ended up getting Kudu and Waterbuck. Kudu is a must and a very special hunt, especially if you can get up into the Waterberg mountains.
I was initially not interested in waterbuck until I saw a living example- a very majestic animal and great fun to hunt- but tough- their gray fur blends right in with the bush.
I'll get a nyala next time!