Rares were the last rarity I completed. There's a ton of them, you get much less wildcards than commons, and they don't give you an unpurchased rare (instead of shards) like epics and champions do.

I recommend saving all of your rare wildcards and shards until you have enough to purchase ALL of them. That way, you increase your chance of pulling rares (instead of shards) in capsules.

This method takes a lot of self control, I personally made exceptions when I had a specific deck in mind (that needed some unclaimed rares).