I got 2000 Treasure Bags from my Cursed Earth mob grinder, so I opened them all up the other day. Took a while. LOL
I got around 20,000 gold coins (Thaumcraft, no idea what I can do with those), along with a TON of potions, both regular and splash (threw most of those away, pretty useless, so they don't clutter up my AE2 storage). The "Uncommon" bags yielded a lot of armor (threw that away also, already using Flux Armor) and a bunch of spell books. Oddly, every single spell book was "Blast Protection I". meh Threw those all away too since I've already made more than I can use of those Ender Quarry blocks that require spell books in the recipe.
But the reason I'm posting this is because I had a heck of a time finding emeralds in Infinity (like a lot of other people)... and the Uncommon bags provided me with over 100 (!) emeralds in total!! That was a huge, massive bonus and made all the time I spent well worth the effort. Also got a lot of Ender Pearls (have too many of those already) and Gold Ingots (always useful).
I also got one "Rare" Treasure Bag. One out of 2000. LOL Yeah, that's rare all right. But when I opened it, it was a complete disappointment. Just some enchanted low-level armor and a couple of other things that were so memorable that I already forgot what they were. LOL
tl;dr: "Uncommon" Treasure Bags yield the best treasure IMO - emeralds! Unless you're into gold coins, not much else of value in them.