[gashatfreak]

Not exactly sure which is worst to best because I haven't watched a lot yet, but I can definitely tell you some shows from darkest to most light-hearted.

Dark: Garo, Hakaider (a spin-off of Kikaider)
Medium: Kamen Rider, Ultraman, Kikaider
Light-hearted: Super Sentai, Ultraman

Of course, earlier Super Sentai shows were pretty dark (you know Rita Repulsa from Power Rangers? In Super Sentai she is a child-hating devil worshipper known as Witch Bandora, but don't let that get to you, it's explained why she hates kids in the latter half of the show).

On what to avoid, maybe Kamen Rider Ghost? A lot of people on this forum downright hate that show due to lazy writing (but, that's up to your discretion. Personally, I was amused by it).

If you want to get a crash course on Super Sentai, I recommend Gokaiger (a superior version of Power Rangers Super Megaforce), which introduces you to the first 34 Super Sentai that came before it.

If you ever get confused as to which show/character/whatever is which, there's always the wikis.