In my childhood opening any pack was a huge win.
Between 2013-2014 I finally had chance to open few sealed vintage booster boxes, 1st ed. Neo Genesis and unlimited Gym Heroes/Challenge were certainly the highlight moments.
During modern/ultra modern era I haven’t bought too many items for opening purposes but japanese CP6/Shining Legends and english 151 were great sets to experience. 
I’m biased because I don’t care for modern and when I did rip the packs it never felt the same as vintage. My advice is get any Japanese vintage pack. The quality of those cards is so good and it just feels so nostalgic. You might have to stretch the budget slightly but it’s worth it!
Growing up:
Any of the gym sets. Loved the idea of owning a card that was a gym leaders Pokemon…for me it tied in so well to the anime and gameboy games.
Modern:
Maybe unpopular nowadays, but I really enjoyed Hidden Fates. I love the idea of shiny Pokemon and was fun to try and chase down the master set and the setlist was so strong. 151 was another really fun open as well.
Japanese 151 has been my favorite over the past four years. I’m also a dude who loves the original Pokemon, so understand I am biased 