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I recommend Necrons if your definition of easiest includes being easy as sin to paint. They are also solid models without a ton of finicky bits for the most part, so they are easy to assemble too. (with the exception of the ghost ark, that one is a PITA)

Grey Knights are easy to start with if you mean easiest to 'afford'; as the elite army of elite armies, their extremely low model count allows for some really cheap purchase costs.

And finally GWs' lovechild: The Ultramarines. These guys get all the newest stuff and attention to detail, as they are the flagship for the 40k product line. Go with them if you want a solid army with lots of rules support and options.