LogFAQs > #969946082

Honestly...any continuity between the series is very, very loose at best so you can really jump in at any point and choose based on whichever series looks/sounds appealing to you. My personal recommendations:

Gaim - Rival dance crews battle for clout using weapons (based off of fruit) developed by a shady corporation. The whole thing eventually unravels into something much bigger. This one was written by the creator of Puella Madoka Magica so that should tell you what time we're on.

Build - An AU Japan that got split into 3 warring regions due to an alien artifact from Mars. Main protagonists are an insomniac, eccentric genius and a boxer who escaped from prison. Has strong themes about politics and war and the cast is very likable.

Ex-Aid - Doctors fight a deadly virus using video games. Yes, literally. But it has one of the most memorable casts in Kamen Rider history and it's just so off-the-wall ridiculous that it turns around into being awesome.