Here's my list:

Southern Impala
Common Reedbuck
Common Waterbuck
Southern Greater Kudu
East African Greater Kudu
Gemsbok
Nyasa Wildebeest
Lichtenstein's Hartebeest
Red Hartebeest
Warthog
Steenbok
Burchell's Zebra
Hartmann's Zebra
Hippopotamus
Cape Buffalo
Lion
Leopard
Elephant

And I, unlike some of you guys, definitely DO want one of each!

But only one plains game animal for the rest of my life?

Keeping it affordable (i.e., ruling out the exotics, like the Lord Derby's eland, mountain nyala, sitatunga and bongo), I would have to agree that the southern greater kudu would be the one.