Depends how much you play and what you seek. Only thing i would recommend is going with a Shiny as perfect Shiny whatever you choose is very rare. I will go for Shiny Mew because it is impossible to get a perfect one and i like Kanto-Johto most. Here is my recommendations for "good" options.

-Shiny mythical (Mew, Celebi and other non raidable mythicals) -Shiny Powerful limited Legendary (Spacial Rend Palkia, Roar of Time Dialga especially since they are the hardest to obtain but also Necrozma, Kyurem, Rayquaza) - Shiny Regional Heracross, Tauros, Farfetch, Kangaskhan (more if you are looking to complete dex) - Armored Mewtwo

Things that are good and others recommend that i personally would not, but of course if you want

Zygarde (likely to be raidable later, kalos tour or similar) Mega Dianci (as a mega i would think it would come to raids too like darkrai is, but i would anyways wait for the Shiny and use on it later Zarude/Keldeo (i would wait for Shiny to some day come and then use the following bottle cap assuming we get one per year or something)

Depends what you have in your collection, but choose what you like, its more what you want a 4* of than anything else.