Going by the numbers in the
Combat Explained thread, assuming:
Maceman, City Raider 2
Aggressive Maceman, City Raider 2 + Combat 1
Musketman, Combat 1 + Cover
Aggressive Musketman, Combat 2 + Cover
Samurai, City Raider 2 + Combat 1
Knight, Combat 2
Conquistador, Combat 2
Longbowman, City Garrison 2, fortified, no hill, no remaining city defenses
All units undamaged
Matchups:
Maceman: 8 attacking 9+1FS = ~20% chance of victory
Aggressive Maceman: 8.8 attacking 9+1FS = ~30% chance of victory
Musketman: 9.9 attacking 10.2+1FS = ~30% chance of victory
Samurai: 8.8+1FS attacking 9 = ~45% chance of victory
Knight: 12 attacking 11.7 = ~60% chance of victory
Aggressive Musketman: 10.8 attacking 10.2+1FS = ~70% chance of victory
Conquistador: 12+1FS attacking 11.7 = ~70% chance of victory
Analysis:
When everything is fresh-built, Aggressive leaders get the most
against Longbowmen out of Musketmen, while everybody else - especially Isabella - gets the most out of Knights. However, Knights become marginalized if there's a Pikeman in the city (which Macemen/Samurai can virtually ignore, and vice versa when Crossbowmen are among the defenders). Macemen and Samurai do better than the chart when both sides have 3 promotions, since City Raider 3 keeps up with City Garrison 3 a LOT better than tacking on an extra level of Combat. What's more, Macemen and Samurai upgrade to Riflemen not too long after getting Gunpowder, AND those Cover Musketmen become utterly useless once the enemy starts fielding Musketmen of their own.
Conclusion:
Remember that you probably won't have gunpowder until a LONG time after you get Civil Service, so for most of that era, you need Macemen and Knights against the big cities ("big" as in "need Catapults to take"), while just Knights can run around on the flanks overrunning the more lightly-defended cities and pillaging stuff.
Despite these numbers, Musketmen are largely irrelevant. They come too late to make much impact on medieval-era battlefields, especially if the front is a long ways off, and even if they DO show up in sufficient numbers to be anything more than a novelty, they'll just be replaced by Riflemen in a few turns anyways...and you
really want those Riflemen to have City Raider promotions

.
The only exception I can see is France, with their 2-move Musketeers. Louis can send them along to escort the Knights (since Knights can't get defensive bonuses), and Napoleon can use them as mega-Knights, slaughtering
everything with them until they become obsolete.
Long story short, Musketmen are more potent than Macemen and sometimes more potent than Knights, but unless you beeline straight from Guilds to Gunpowder, they come too late to be much use. Using Musketmen as your primary attackers will also sacrifice a lot of your potential offensive power when Riflemen come along, since Macemen can upgrade to Riflemen, taking their City Raider promotions with them.