Hey there!
I would start with the Daily Gem pack, that's 10 bucks for 2000 gems montly. Best value in the game.
I also am partial to the Forge Passes, which end up dropping a ton of glyphs in addition to craftable 5-6 star gear.
After that, I would look for Warrior's Circle packs, which pop up 30 days after your last purchase, and then rotate through several different options. I am fond of the ones that contain energy and refill bottles. After that, the ones with books....although I see books as things you get from CB and missions.
5 star chickens are an okay purchase at first glance, but buying energy lets you farm silver at the same time you are making 5 star chickens in 12-3 campaign. And it lets you farm gear and masteries. Gear and masteries and food for 60s are the three big things to advance your account long term, and progress into NM clan boss where you will get books and shards.
After that, there's occasionally a pack called Big Sack of Gems that is a pretty good deal.