In Warhammer 40k I believe the Soritas and Imperial Guard are both overall far more powerful than the Space Marines. The reason for this is simply put support and numbers. Individually there is little contest (except for a few of the heroes and such from the respective units) but considering the relatively limited numbers of space marines and the fact that they basically provide their own logistics (coming from a single planet from which they draw chapter members in most cases), if say someone who managed to send out a communication on that level and ordered either group to basically kill all the Space Marines I'm pretty such they would (eventually) succeed. The Imperial Guard probably having 4-5 decent tanks for each Space Marine, and the "Sisters Of Battle" having the entire uber religious network behind them. Even if the Space Marines were to wipe out entire planets full of these guys they simply couldn't manufacture bullets fast enough to kill them all...
As far as IRL military forces go...
I'd say probably the US Navy SEALS or US Army's Delta Force. In general it's our morality that holds our guys back, not lack of capabilities or anything else. A lot of the military forces that outperform them in a lot of the training (like British Royal Marine Commandos) are simply weighed down by less BS because their goverment isn't *quite* as messed up as ours.
I consider Spetznatz to be pretty tough overall, but in the end I think they are inferior to our special forces except for the simple fact that they work for a nation that has little in the way of combat morality and tends to be famed for it's "Git 'ir done" mentality which the US has been increasingly lacking.
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Historically, I'd probably say the most BA groups would have to be found among Christian knights, probabl an order like The Templar or something similar. I say this because all politics aside, they pretty much managed to enter The Middle East, whoop up on everyone, conquer the Holy Land, and gain vengeance for slaughtered pilgrims, with no real support or logistics from anyone. They also held out for a very long time comparitively speaking.
The Crusades having been fought accross a tremendous distance for the time period, where supplies and men weren't even guaranteed to make it accross the ocean (either on the way there, or back). They were literally outnumbered hundreds or thousands to one by other religious fanatics, and they still tended to win. Ultimatly being forced back because they were in the middle of enemy territory, and every knight that fell was more or less irreplacable where there were always more screaming Muslims to replace them.
History doesn't like to present it that way (reinventionism, political correctness, etc...) but there are reasons why the alleged "winners" still have a bug up their collective butts about it today.
Many comparitive forces like The Spartans and such performed well in some nasty battles, but generally didn't have the same logistics problems (the 300 was a suicide action when you get down to it, so logistics weren't a factor for that reason).
When I say "Crusaders" keep in mind I am talking about a combined European force here though (held together largely through religion) not the force of one paticular nation. The Crusades being one of the few things that has gotten all of Europe working together, and the results were absolutly terrifying when you get down to it. One of the more interesting things about the more successful crusades (there was more than one, and several were less than resounding successes) is that nobody should have ever been able to achieve that under any circumstances given the tech level.
This is incidently why when people compare say a European Knight to a Samurai or whatever I tend to think it's a bad joke when the conclusion comes out in favor of the Samurai or whatever.