As I explained somewhere on this forum a couple months back:
Walking tanks, regardless of the number of legs, are insanely impractical bits of lightly armored, low mobility, and light-payload targets.
In real world physics, just about any country's Main Battle Tank (American Abrams, Isreali Merkava, Russian T-90, UK's Challanger II, the German Leopard) is going to easily handle any mech, HERC, or Gundam you can come up with.
Building up means exponentially increasing weight, which greatly reduces payload, stability, and how much armor you can slap onto it. Since they walk, they have joints. Joints are ALWAYS a failure point, not to mention weak points in already light armor. You cannot arm them with heavy weapons systems (GAU-8, any MBT's main gun), which greatly reduces range and firepower. Because of their complicated drive system (which adds yet more weight, and more points of failure), they would have extremely complicated control systems that you would have to spend a great amount of time teaching to your pilots.
If it loses a leg, pilots cannot get out and crack track.
Really, the only plausible Walkers? Are really just armoured exoskeletons.
Also, robot is creepy. Also