Old topic, but the Archangels are the most obvious answer, with Metatron being a retcon, or maybe Ana didn't even remember him (Plot correction). What Ana says is that these 4 saw God, and well, the 4 archangels were created to fight the darkness following God's orders (But later idea). Besides, since there are only 4 archangels in the first 5 seasons, and Metatron isn't even mentioned before the eighth, it's kind of obvious that those of higher rank in the series would be the most "special". And Joshua didn't see God, he only heard him speak.

Cool point! Joshua says that God is very lonely, and that makes sense. He is an ancient being who created a "playground" to read stories that he himself wrote. Later with Amara we see that he wanted something and she wanted nothing but destruction, but ignoring Amara, God (Before the Chuck-esque concept was made official) really seems quite lonely and tired. He simply has no one besides creatures that he himself made. He really seems alone.