What also makes their relationship so great is that Megatron is very aware of Soundwave's loyalty and reciprocates with an equally strong impression of trust. There is no Decepticon that Megatron relies on as much as he relies on Soundwave. Be it for advice, technical support, making energon cubes, research or anything of the like, the Decepticon leader always had good use for Soundwave's abilities. Megatron must trust his subordinate quite a bit to depend on him so completely.
Another interesting point of their relationship is the amount of respect Megatron seems to have for him. The way he treats Soundwave indicates that he sees his subordinate as more than just a useful tool. He openly acknowledges Soundwave's opinions even if they differ from his own. For example, in "Masquerade," when Soundwave pointed out that the systems were unstable and suggested abandoning it, Megatron acknowledged his worries, even if he didn't change his course of action. He may not follow all of Soundwave's advice, but at the very least he acknowledges, which shows that he respects his subordinate enough to validate his findings and suggestions. Even in the very rare moments when Soundwave has a lapse of brain function, Megatron doesn't belittle him as he would Starscream. For example, in "City of Steel," he clearly saw the creation of Optimus Prime's clone and yet still thought it was the real thing. Megatron didn't laugh at him or criticize him, instead he immediately quieted his subordinate's fears by informing him that it was, indeed, just a clone. Also, to my knowledge, the Decepticon leader has never raised a hand to Soundwave. In the episode "Insecticon Syndrome," when all of the Decepticons except for Megatron are under the influence of Bombshell's cerebral shell, Megatron was quick to shoot the Seekers. Yet, even when he was within striking range of Soundwave, he never attacked him. This is an almost vanishingly minor point, but one I had to make nonetheless.
Ultimately, their personalities and abilties compliment each other. Where Soundwave lacks in physical prowess is made up by Megatron. Where Megatron lacks in technical prowess is made up by Soundwave. Megatron's great leadership is rewarded with unfaltering loyalty. Soundwave's integrity and intelligence is met with trust and respect. Both share a quest for knowledge and determination for the Decepticon cause. They, along with Shockwave, are the backbone of the Decepticons.
