As lucapal says, if you give us a little more information we may be able to give a better answer. Your status says you're from the US so no matter where you go the flights will be expensive for a start.
If you like the ocean then Mozambique may be a good option, beautiful coastline, an interesting African/Arab/Portuguese culture, great diving, Whale Sharks (during the right time of year) and Gorongosa NP for the interior.
If you like culture Mali is certainly a great option, fascinating cultures, great Sahelian scenery - if you're into that kind of thing the boat trip from Mopti to Timbuktu is fantastic, just watching the world go by along the banks of the Niger river.
If you want a mixture of wildlife, some culture and beautiful beaches Sierra Leon is a good option - less infrastructure than some of the other places but definitely a place to go sooner rather than later as it is a jewel slowly being discovered.
Rwanda is also fairly small so easy to get around, great for the Gorillas ($500 for a permit to go see the Gorillas mind you, and that's per person, and you get to spend 1hr with them), also Chimps (as in Sierra Leon) and lake Kyvu. I'd probably suggest combining Rwanda with Uganda if that was where you were going to go - unless you were only going for a very short time in which case it's a long way from the US.
Australia, obviously, is the odd one out being a different continent, different culture, beaches, outback, jungles in the north, deserts, great barrier reef..... Huge place with lots to do but also very expensive
Senegal also has interesting culture, some natural beauty and beaches - the beaches are possibly more developed (it can be a beach destination for some Europeans, especially francophones)
Nigeria I can't comment much on though I've only heard good things from people who've been (though not from the media but that's a different story).
Of the above I've been to Mali, Mozambique and Rwanda, and have done a fair bit of research on Sierra Leon. Of the places I've been, Mali and Mozambique are both fantastic and Rwanda is where I've had the best experience I've ever had travelling (the Gorillas). Mali has had some political problems recently which is something to bear in mind. As to which to chose, well again, depends on your interests, budget, time frame, and language (which may or may not be a factor)
