Which Kamen Rider Series/Book should I Watch/Read first?

my nephews like 555, agito, kabuto, the one with the mirrors, and the one where he can change colors

Unsure.... I tend to follow Super Sentai more than the Kamen Rider. Like most things I tend to start at the first and work my way through just because I like being geeky.

Isn't that basically power rangers?

Showa-wise, I love V3. It seems like he was constantly performing crazy stunts and one-upping Destron. Of course, all Showa shows have a completely different flavor to heisei shows, one that some people just can't deal with or, at the very least, take some time to adapt to.

Heisei-wise, I think W's a great series to start with, if you like a bit of light-hearted mystery. If you like a darker show, go with Kuuga, Agito or Faiz. Specifically, the Agito or Faiz if you want to see likeable characters put into horrible situations, and Kuuga if you want to basically see monster serial killers running around racking up some extraordinarily high body counts. I think Agito's my personal favorite, though W and Faiz both come pretty close. Wizard's pretty good so far, but we'll have to see if it manages to keep it up.

As far as shows to skip... I like Decade, but it's certainly one of the weakest. Kabuto's also weak, in that the plot manages to basically implode and the writers have to do an ass-pull, but it's got better/more interesting characters.

Still, if you have the time, I'd say at least try them all.

555 and Kabuto are my favorites in the Heisei Era.
Before that Kamen Rider Black and One.