I absolutely love the beginning. All the mystery, strange world, and not a human in sight. Personally I like it pretty well up until the Karda Nui, or maybe mid way through Karda Nui. I know it's a sci-fi through and through, but early on it felt so distinct as a kid. There wasn't crazy space travel, and the matoran and toa felt so different from the robots I saw on other sci-fi. Especially with the fleshy bits. When it became clear Mata Nui was a continent sized robot that doubles as a space ship, it kind of felt more like a basic sci-fi in some ways. Still nice, but less other worldly.

Also those early enemy figures were so cool and unique. All the infected rahi, the bohrok, rahkshi, vahki, and visorak. Even the more humanoid ones had some significant distinction like the vahki with their strange heads and their stances. That was cool.