I was going to recommend Kuuga through to Blade. Those were the best years in my childhood. Even watching them as I grew up, they're still great, so I disagree about the whole "not ageing well" like someone here mentioned with Ryuki, Faiz, and Blade. They're all great shows. Faiz is one where things get pretty grim dark, but if you don't like that, you can tone it down with Ryuki, and if not, Blade is the least darkest of all, but it's ending will leave you in awe.
Agito on the other hand, was considered one of the best series around, but since it's an indirect continuation of Kuuga, I recommend Kuuga first, not because the plot connects or anything, it really doesn't, it's just one of those things you'd want to keep in mind that "Oh, so that's where this came from", but even then, there aren't much to really connect.