I have to mention this. The latest update of the Metal Slug Defense (v1.8.0) should have re-shuffled the cards. Now, the strongest yet the most cost-effective unit has appeared. No doubt, it is no one else but the Morden Robot.
Here, how do I judge which unit is the strongest?
Cost EffectiveI am not talking about the clunky unit such as the Jupiter King (it costs 900 AP) which is summoned during the late game. Instead, the cost and the production rate are the main concerns here. Morden Robot, at the cost of 140, is able to serve immediately at the early game after several AP speed upgrades. I would say he is one of the most effective units when playing the Wi-Fi VS mode.
Outstanding Hit Points and Attack PowerI haven't had the chance to make it 1 VS 1 with the other popular units (such as Trevor, Ralf, Leona, etc). But from the observation so far, the Morden Robot is the only one who can defeat most of the stages in the World 3 ALONE! (Seriously, it works in the Final 'Hell' stage of the Emergency Area North Asia N. East as well, coupled with the Metal Slug Attack) With the 4000 HP at the level 20, he simply fears no one at the early game. And to my surprise, the Morden Robot alone is capable of beating the BIG SHIEE in the final stage of China!
Special Ultimate SkillMost 'zombie-d' units have the special feature of 'Not regress-able' when they are being hit aggressively. However, the Morden Robot makes this feature to another whole new level - 'unaffected At All'. What I am trying to say is that, the Morden Robot is unstoppable once he is summoned. No one could take a buffer of rest when fighting with him. Not even the strikes from the Jupiter King or the Big Shiee could push him backwards. Moreover, he is able to cause damage after the death! (hurls his robotic head and follows by a suicide attack)
Combining these factors, I hereby can conclude that Morden Robot is really the strongest unit so far who can contribute significantly in your battles. In case you are wondering, the Morden Robot is un-lockable through the POW rescue in the Emergency Area 'East Europe' of the latest World (World 3).