It really depends on what you're looking for.
If you just want to open packs and see what you get, buy whatever set you'd like. If there's a set with a lot of cards that interest you, go with that set. If you want a little bit of nostalgia, go with the Mythical Pokemon collection.
You really can't go wrong getting whatever pack you'd like if you're just collecting. That's all that I do with the TCG these days is just collect the cards. All that I buy is just whatever package I'd like to get at the time. Though I also keep my eye out for any packs with special pins of Pokemon that I like in them, since I like collecting those.