>>34373984Fire Red / Leaf Green, then Heart Gold / Soul Silver.
After that you can kind of explore the series at your own pace. I'd suggest Black / White, then Black 2 / White 2, then either X / Y or Omega Ruby / Alpha Sapphire. You can skip or play Ultra Sun / Ultra Moon, there are fun monster designs in those games but it plays a lot like it's immediate predecessors in Gen 6.