NASA has been the greatest space agency that ever existed, they explored Mars with Viking, landed men on the Moon, explored the outer solar system with Pioneer 10 & 11 and Voyagers.
However NASA may not be king forever, remember the Russians already gave NASA a run for their money, and the early Soviet missions were way ahead of the USA lauching the first man in space, first spacecraft to another planet, first woman in space, first spacewalk. NASA won the spacerace but today it is in a bit of a mess ( budget cuts, CEV LOX/methane propulsion dropped, Dawn cut.... ) ground based scopes like Keck also got cut and NASA have to get Shuttle flying again.
To start the debate I'll give you a list of my top five
Russia : The Russians already have great experience from the Soviet-era, they were an extremely worthy opponent and they have done things that NASA never got to do. The Russians landed the Veneras on the hellish planet Venus, they explored the Martian Moon phobos. The Soyuz and Progress spacecraft have been fantastic workhorses for Russia's space plans. Russia built many manned space stations, done many endurance and biosphere experiments and have been in space for twice the amount of time NASA has, they built the Energia launcher and put robotic landers on the Moon and explored the Venus atmosphere with balloon craft. If you were to ask 'Which rival could go put people on Mars in the next five years given the political will and funding' ? One would have to answer 'its the Russians'.
Europe : For a while Europe's space exploration seemed a bit of a joke, it was full of quarreling bickering nations who were all trying to set up their own launch pad in various places ( Italy in Kenya, British in Australia, France in Algeria ) and Europe's space plans were pulled in different directions. Then the ESA came about in the late 70s, they done a number of good missions like Exosat, Giotto to the Comets, Cos-B. They have recently started to announce their ambitions plans doing joint missions with NASA ( Cassini-Huygens to Saturn ) and RKA-Roskosmos ( Soyuz-Fregat launch of Venus Express ), along with many of their own home grown missions. After NASA the ESA have the next largest budget, it is supported by many giant economies in the EU, and the ESA also consists of non-EU countries ( like Norway & Switzerland who are involved in atom-smashers like CERN ). If you check out all the robotic spacecraft the Europeans are building, and exo-planet missions in the works you'll get the feeling these guys are going to dominate unmanned space exploration, they also announced plans for Mars.
Japan : The Japanese have made great contributions to space exploration they launched the Sakigake probe, launched Astro-B ( Tenma ), did the Yohkoh Solar Telescope, launched Astro-A ( Hintori ) and sent Suisei to Halley's comet. Early Japanese exploration was supported by Japan's strong industrial powerhouse and large economy. Today Japan's space journey has been very stop-start, Japan has a lot of debt and is no longer the top economic power, it ionce had a mighty economy but today is weak when compared to the EU or USA and might start to sink behind China, Japan could be the place of the sinking Sun. Japan's space exploration troubles seem to have come about during the time their economic bubble burst, sadly the sun may be setting on Japan's space plans but they might also make a good comeback.
Canada or Brazil : The Canadian space agency ( CSA or ASC ) when compared to others is a small agency with a modest workforce and low space budget. Canada has done a number of Earth observation satellites, and enviornmental satellites. Although they never got spaceport Canada going thanks to their joint missions with NASA there have been a number of Canadian astronauts in space. Canada have built robotic arms for the Shuttle and ISS and it seems to get the maximum out of its small space budget by doing partnerships and collaborative programs with other agency's and countries, such as NASA, Russia, ESA and NASDA/JAXA. Canada achieved some kind of special status as a cooperating state with the Europeans, it is now taking part in ESA's activities and Canadians are in a bid to receive contracts to work on European programmes. Two recent Canadian missions to keep an eye on are its MOST space observatory, and Cassiope the Canadian ionospheric Polar Explorer.
Brazil on the other hand has built its own space port, it had an inital setback when technicians were killed in an explosion but made a successful launch in 2004. The Brazilian Alcantara complex is in an ideal location for launching and tracking geostationary satellites, they are now securing a future by offering the site interational co-opertaion with international agreements, they have also been helping with the International Space Station.
Upcoming rival : The Chinese
China has a large manfuacturing sector, huge work force, lots of rescources and is growing rapidly and is going to become a superpower. The reason I rank the Chinese so low is because they have yet to do much in space exploration and who knows - maybe tomorrow some Chinese leader might declare space exploration to be a waste of time ( although I think its highly unlikley ). China is moving very slowly in exploration but they key thing to note is that China have a history in thinking in the long-term, they are not planning on racing like two Russian and American dragster vehicles which race each other and then suddenly run out of steam. China's space exploration has not been at a rapid pace but if you examine carefully they are making key advances with each step they take into space setting themselves up for something massive, they are building a knowledge base and laying the foundation for something that could be bigger than the first Transcontinental Railroad in the United States or the Great Wall of China. They are setting themselves up with a very good manned spacecraft, a series of medium to large launch-vechiles with very heavy lift in the works and 4 major space ports. Chinese key launch sites will be Jiuquan in the North-West of China, Xichang base in the South of China, Taiyuan in East-China and a new heavy launcher on Hainan island. Bush tried to do some surveillance on the island in April of 2001, but a confrontation occured where a Chinese pilot died and 24 US crewmen were captured during the spy plane incident. Don't be too surprised if in 2019 when the USA is doing a mission to mark Apollo, the Chinese have already got to the Moon and claimed its resources or Mars is called the Red-Planet for more than one reason.
I should also mention Ukraine, the Private-groups and Private sector within the USA ( Space-X, Virgin, Canadian Arrow, Spaceshipone ), India and Isreal. Someday all these smaller groups might become serious space-powers but currently I don't think these guys have what it takes to play against the big-boys.
What do you think ?