A few thoughts
SWAT vs. GSG-9
Why would they be fighting? Mistaken identity?
Jesse James Gang vs. Al Capone Gang
If Capone's guys have automatic weapons there's a brick on the scales. Like the Taliban vs. IRA there's a lot more to it than just the hardware.
Remember, most of Capone's men were thugs. They roughed people up. They assassinated. They drove by. They massacred. They got into streetfights. Sometimes they got into battles with the police. Their tactics, such as they were, were optimized for built up twentieth century cities. In that environment I'd give them the edge.
The James brothers and their men were a whole different sort of trouble. There is no way Capone's men could have touched them for long range shooting. They all grew up as marksmen with the rifle and very comfortable with the pistol. Most of them were Civil War veterans with years of unrelenting battle and hellish combat. Frank and Jesse rode with mother****ing QUANTRILL fer crissake. There wasn't anything they didn't know about keeping their heads under fire, volley fire, irregular warfare or close and dirty fighting.
Anywhere outside the city, especially if there's cover or the fight starts at long range advantage the James gang.
Roman Centurion vs. India's Rajput Warrior
That depends so much on which period we're talking about that it is hard to even speak intelligently about it. The Roman soldier wasn't much on individual warfare or dueling. His strength was the legion. Of course, Rajput fielded professional armies for centuries. Are we talking Moghul period cavalry? Sixth century tribes? The armies that fought the Turkic and Afghan invasions of the twelfth century? Arms, armor and tactics varied wildly over the millennia.
Somali Pirate vs. Medellin Cartel
Puh-leaze. A bunch of guys with AK-47s and RPGs in tiny boats vs. guys who can bribethe military forces of small countries and own submarines, aircraft, armored vehicles and the best weapons money can buy.
Vlad the Impaler vs. Sun Tz
One spent his entire life fighting against a vastly superior army. The other possibly never existed. And if he did his career was mostly as a civil servant, possibly as a commander, never actually fighting. Army against army? Good question. Man to man (assuming Sun Tzu really is a historical character)? Not even worth discussing.
Comanche vs. Mongol
If the Commanche has access to repeating firearms or the fight is on foot, advantage to him. Otherwise it's going to be a really interesting fight. If they have horses and bows advantage to the Mongols. Hell, at any time and anywhere the terrain and climate permit him to use the horse, the bow and the lance without coming up against guns, he's nearly unbeatable, especially in groups. If it gets to knives and wrasslin' it will be really something to see. Old accounts say they weren't great swordsmen on foot. But they have always been big, strong and fanatical about their wrestling including wrestling with knives.
KGB vs. CIA
Navy Seal vs. Israeli Commando
Wonder Warthog vs. Wonder Warthog. Similar armament. Similar (exceptional) training, morale, professional skills.
