Attention, soldiers! The following article contains significant spoilers and/or ending details of Valkyria Chronicles. If you don't want to get shot, tread carefully!
Ace lists are below, don't say we didn't warn you.
Aces are elite units who often drop unique Imperial weapons (typically short-ranged and inaccurate but more powerful than the player's weapons of similar level), high-end vehicle parts, or plans to make improved weapons. They are visually distinct (usually slightly modified enemy Leaders) and are much more difficult to take down, usually having boosted health coupled with high evasion ratings if they are infantry. It is common for Aces to also be Leaders, but they do not have to be.
One practice commonly used to take down infantry Aces is "turning" them: this involves having one soldier run past them so they will turn around while delivering Interception fire, then having another soldier approach them from behind to deliver a killing blow. It is vital to approach them from their rear arc and making sure "surprise attack" is displayed before firing, as if this is not done they will use their normal evasion chance.
All Aces in this game are Imperial soldiers, though Secret Agent Saki appears to be an undercover agent actually employed by some country in the Far East.
Something of note is that several Aces encountered throughout carry modified weapons with adjusted stats compared to their standard Imperial-only counterparts. The stats of the weapon they unlock do not necessarily match the one they use: indeed, some such as Cova the Peacock do not even unlock the same type of weapon they use (in his case he uses a VB GW 2 general-purpose Mortar but unlocks a VB GW 2B(e) anti-tank Mortar).
There is also a pseudo-Ace, Major Burke, who commands a Heavy Imperial Tank which uses his name as the unit name, but he is not counted as an Ace kill and does not have an item drop.
DLC weapons are not required to earn the "Excellence in Armament" Medal.
The DLCs "A United Front With Squad 7," "The Two Valkyria" and "Edy's Advanced Ops" do not contain Aces.
Almost all known Aces are male, in line with the Empire's stance on non-Valkyria women serving in combat roles. The only two cases of Imperial Aces likely to be female are Nora the Tigress, a Lancer in the first game, and Ette of the Icefield, a tank Ace from Valkyria Chronicles 4.
There is also one GRA Ace likely to be female, Cindy Squall/Tempest/Galeforce/Windstorm, a recurring Ace in Valkyria Chronicles 2.