Definitely Build.

Den-O is Den-O. It's mainly entertaining simply because of the Imagin. The story is pretty much nonexistent and what little story it does have isn't very convincing. But... the Imagin are in there. If you like them, you will get all the entertainment you need out of this show. But if you don’t - then there's nothing to see here really.

Build on the other hand is, in my opinion, the single best Phase II Heisei show. It has everything. A unique setup, really great cast of characters, awesome action scenes... and Build really excels at putting these elements together.

I recommend watching Build first, but Den-O is definitely worth watching too. Because the Imagin are hilarious and I love them.

Den-O is a lot of fun, although it’s hard not to argue that it started to wear out its welcome eventually. Build is like a better Kabuto to me: it’s a good show, but I tended to forget about it shortly after it ended.

If you want something relaxing and primarily comedy that doesn't care much for ongoing plot, go Den-O. If you really want the twists and turns of a mystery with great characters, go Build. There's plenty to love about both, neither is a *bad* option.

When it comes down to deciding what series to watch next, I usually watch the first 1-3 episodes of each and consider which one has me hooked on seeing whats next.

I know this is unfair to series that take a while to "build up", but because these are self contained series, that energy makes it easy to binge through a huge portion of the show right away

Actually, I think I'm going to go Den-O first. Why? I have no doubts that Build is the better show...but reading you tell me to watch it first because of that makes me not want to :lol

JK, but not really. It was one of those cases where I knew which one I wanted, but didn't fully realize it until somebody suggested otherwise. I'm curious about Den-O because it's referenced so often. It had such an impact on the world of Kamen Rider and changed the direction that the series went in, so I kind of want to see where that all started.

Besides, if I just wanted to watch the best ones, my KR watch history wouldn't be: Kiva, Drive, Ghost, Ex-Aid, Amazons, and 15 episodes of the original :lol

I mean Den-O is still cool and I'm very much a defendant of not going by what's “best” or what people suggest you “should” see first but instead picking what you actually want to watch... but you still asked :p

But Drive and Ex-Aid are two of the best ones!

In ten years from now, Oma Zi-O should come back as an antagonist to menace that year's anniversary rider on the sidelines, before coming around and teaming with both that title rider and normal Zi-O at the end of some story arc.

Or, on the other hand, wouldn’t it also be pretty great if we never had to see Sougo ever again?

Well, I would be fine with just Oma Zi-O (The seventy-year old overlord, that is).