He is most famous for his poetry and I recommend that. I think if you want to read more abstract or metaphorical stuff then Sonnets to Orpheus is the best bet. They are sonnets (obviously) and aren’t difficult reads. But if you want something more, um, real (I guess), then you should look at the Duino Elegies because they deal more explicitly with the War. Of course he only experienced the war obliquely so his poems re that aren’t as critical as, say, Sassoon (not the hair guy) or Wilfred Owen.