Zone of the Enders for the PS2 and its sequel Zone of the Enders: The Second Runner are probably the two best mecha action games yet made. While this is in part due to the game's cool mechs and world, as well as to the incredible production values Konami put into the two games, the ZOE games earned their reputation thanks to their excellent gameplay. The action in the game is extremely fluid and fast-paced, thanks in large part to an excellent control scheme. It is in the area of game control that ZOE and ZOE2 really shine.
The genius of the Zone of the Enders games comes from the context sensitive input. The lion's share of combat options can be performed with just the Left Analog stick and two buttons. Obviously, the left analog stick moves the main mech, Jehuty, around. The real brilliance lies with the square button, which causes Jehuty to both shoot at the enemy at long range and use melee attacks if close to the enemy. Similarly, the R2 button can either cause Jehuty to dash if Jehuty is moving, or enter Burst Mode if Jehuty is standing still. So, with just two buttons and an analog stick, Jehuty can perform high-speed maneuvers and execute fancy melee combos, a dashing slash, a powerful Burst Slash, rapid-fire basic shots, a homing dash shot attack, and the Burst Shot. This gets a lot of mileage out of just a few controls, freeing up other buttons to be used for different things. Most importantly, fighting quickly becomes very intuitive.
At this point, I will draw some comparisons with a few other action games, mecha and otherwise. Most other action games I have seen tend to designate melee attacks and shooting attacks to separate buttons. For example, Gundam vs. Zeta Gundam, a decent, yet flawed, mecha action game for the PS2, uses the Square button for gun attacks and the triangle button for melee attacks at all times. In Gundam vs. Zeta Gundam, each mech has multiple elaborate multi-hit melee combos that can be pulled off by pressing the Triangle button and pressing the Left Analogue stick in certain directions at the same time. However, there are a couple major flaws with this system. First off, it is possible to execute a melee attack even at long ranges, where the attack can't possibly hit and can potentially leave the player vulnerable for several seconds as the attack animation is carried out. Even more problematic though, is that it is really hard to connect with these attack chains. These melee attacks target the area directly in front of the mech, no matter where the enemy is actually standing (which is pretty common for melee attacks in actions games). Because of this, it is very easy to miss a moving enemy, or only catch the enemy with a glancing blow. Being locked on to an enemy doesn't have any effect on melee targeting.
What makes melee combat work for ZOE is that lock-on does matter in melee combat (heck, you need to be locked on to an enemy in order to use melee attacks). This is something that can be seen in other really good 3-D action games, such as the Legend of Zelda and Devil May Cry games. In both series, it is possible to lock on to a single opponent. In that mode in both games, the player character is either always facing the opponent, or always re-orients towards that target the moment before launching an attack. That way, there is little chance of accidentally missing the target, even with melee attacks (unless the enemy can dodge quickly of course). Zone of the Enders takes this one step further. Not only does Jehuty always orient towards it's quarry, it will actually move towards its opponent and home in on it with melee attacks, particularly the Melee Dash Attack.
In short, the brilliance of Zone of the Enders comes from building combat around a lock-on system that reads the distance between the player character and its opponent. At that point, the context-sensitive controls kick in. As the player closes in on a target, the system automatically switches over to close-range combat mode and starts to judge the distance between the player and target to determine if the mech will need to close in as part of the attack or not. As a result, the controls are simple, and Jehuty always does exactly what the player intends, instead of stupidly attacking thin air.
Another major innovation of Zone of the Enders is making 3-D space combat more managable by giving the player easy to use altitude controls. Many mecha games, such as the aforementioned Gundam vs. Zeta Gundam overlook that simple detail, resulting in games where the player can get stuck in awkward places as the camera shifts around. While the ability to rise and fall is typically not needed during combat in Zone of the Enders, it does come in handy. Any game with free 3-D movement should give the player all three dimensions of movement controls (a jump button doesn't count if there is no gravity).