If I'm going just by sounds, Kabuto, though that's a full package thing. All the sounds on that suit complement each other. It all sounds like a piece of tech. Meanwhile, belts like Wizard's are so disjointed when it comes down to just sound.
Now, if it's just henshin sounds, early Kuuga when he punched to henshin. I prefer the henshins that don't interrupt the action and since there's punching sounds I'm counting it.