There are many types of terrain to fight on. Who does the best on which terrain? This is perhaps something to keep in mind when you need to figure out who might have an edge in combat.
For example, in deserts, the fights are likely to be between tanks. There are armies with a lot of experience in fighting in the desert; the United States, Saudi Arabia, Israel, and Britain. The Saudis spend all their time in the desert. They even fought reasonably well in Desert Storm. However, the Israelis have fought two major wars in the desert (1967 and 1973), coming out ahead in both wars against numerically superior forces. The British fought well in Desert Storm and Iraqi Freedom. The Marine Corps has also fought in those two conflicts, as has the U.S. Army. But the U.S. Army has the edge, partly because the National Training Center at Fort Irwin, California is in the desert. This superiority has been proven in engagements like 73 Easting, in 1991, and the drive on Baghdad in 2003.
Mountainous terrain is difficult to fight in. First, there are the effects of the high altitude (thinner atmosphere means less oxygen which can cause hypoxia among troops). Next, one has to realize that one cannot take tanks up to that area easily. Helicopters will also have a great deal of trouble (their performance drops precipitously in terms of payload and speed due to the thinner atmosphere). The United States has a division that has been designated to fight in mountainous terrain (the 10th Mountain Division), but real mountain combat has not occurred in a long time. India and Pakistan fought a low-intensity conflict in high altitudes in the late 1990s. The Swiss also operate in high altitudes. The famous Gurkhas are from Nepal, a country that features the highest mountains in the world. The best? Indian and Pakistan will probably be able to fight a major battle (there was a fight between the two countries in the late 1990s that was primarily fought in high altitude), but the Gurkhas will have an edge in low-intensity conflict up in the hills.
The jungle is one of the other hard places to fight. Here, it is not only hard to bring in tanks (if you think a tanks dead zones are bad when you are in the desert, just imagine what it must be like with all the vegetation in a jungle). The Australian Army, United States Army, and United States Marine Corps all have had experience in this area of warfare. The British Army and Malaysians also have experience in this sort of warfare as well. The Gurkhas have some experience with jungle warfare as well. Probably the best are the Australians, but dont count out any of the Southeast Asia armies, who have had experience in low-intensity conflicts in a jungle environment since the 1960s.
Arctic climates are tough to fight in. The issue here is the low temperatures in arctic regions, which cause problems for a lot of mechanical gear. Overheating in the desert has been a more common issue, but operations in cold temperatures are also important to keep in mind. Canada, Norway, Russia, Sweden, and Finland constantly operate in these regions, although Denmark has often put in appearances around Hans Island (a small island whose ownership is in dispute between Denmark and Canada). Probably the best of these forces is Norwegian, which spent years preparing to deal with a Soviet (or Russian) invasion, with Finland a close second. Canadas underfunded military comes in third, mostly because the United States could back up its northern neighbor.
Finally, we get to urban warfare. If you think fighting in a jungle can be tough, cities and major towns are just as bad. Not only does one have to deal with reduced visibility, but its much easier to get to higher vantage points in urban terrain than it is in jungle terrain. This is particularly bad news for tanks and armored vehicles. An AT-4 light anti-tank weapon or RPG-7 that could not penetrate the frontal armor of a tank can get through the top armor, which is usually the thinnest. Russia, Israel, and the U.S. Marine Corps all have had to fight in cities in the last ten years, while the British had to deal with Northern Ireland for a while. The best fighters are probably the Israelis, whose lessons learned have assisted American troops in Iraq. They not only have the experience, but a great deal of equipment designed for urban warfare.
One thing these forces that excel in certain areas have in common is a high level of professionalism. Often, these forces will have trained well, and will have good and well-maintained equipment in large quantities. These are critical advantages in any sort of warfare, and often will determine the winner long before the first shot is fired. Harold C. Hutchison (firstname.lastname@example.org)