>>60549954safest buys is always sealed product, you wont lose money holding booster boxes or pokemon center etbs for 2+ years so if you're thinking long term products like 151, Prismatic, Evolving Skies, maybe Destined Rivals are solid
but that is as boring as stocks and you might want something cool to look at longterm too. obviously I like zards, you cant go wrong with charizard. 20+ years they'll all be worth more.
if you dont like zards my best bet would be cards with "nicknames" see the stained glass birds, moonbreon, lovebirds, bubble mew. basically anything with a nickname that sticks means its popular & in demand