One could assume the Klingons have the largest military force of the three traditional powers. The Federation would be second with the Romulans a close third.
The differences between them are that the Romulans have changed from a pure stealth mode tactics like in the ENT and TOS days to having the largest ships in their fleet. The Romulan warbirds are huge and powerful.
The Klingon have numbers but are at a disadvantage in terms of technology by the 24th century. I think Praxis and the change in their culture has stunted their research and development groups so that they are still using hundreds of nearly 100 year old (or older) starships on the front lines. Though to be fair, so does Starfleet with there endless supply of Mirandas and Excelsiors.
The Federation seems to keep the edge (for the most part) in terms of technology. Especially defensive technology and sensor arrays. There weapons seem impressive as well when used at full potential.
By comparison, the Cardassians, seem to be behind the three powers but able to muster a tighter defense in their own space. Enough to keep the Klingons and Romulans from taking advantage of them until the Dominion inspired political chaos leading to the Klingon invasion of Cardassian space. At which point the Klingons muster a large reserve of ships for war and pretty much crush the Cardassians.
The Federation tends to hold back a lot, and their ships and crews are not designed for only combat. They don't fair as well with certain designs, but others, if properly handled, can make short work of Klingon ships. Federation ships tend to operate alone while Klingon attack groups are just that, groups. So the outnumbered Starfleet ships get mauled during their short war.