what are you stockpiling... don't want to transfer just in case???

Santa-Pikachu: everybody has got them, i do not expect them to be valuable one day. I keep 1 Pikachu and 1 Raichu.

Lickting: Afaik, there is an evolution in a higher PKM-Generation. I keep the high-iv-ones.

Porygon: There is an evolition in Gen2. I keep the high-iv-ones.

Dittos: I keep highest IV, lowest CP and highest CP just for the case, someone will find a use for these. I do not believe, that anybody will want to trade one.

21 Santa Pikachus are way too much, transfer some of them and evolve some. Having Lickitungs is no use for now, but keep an eye out for the next evolution. Porygons are rare, but evolving is probably hard due to items. Dittos are useful when niantic adds the "breeding" feature to the game.

I stockpile high level / High IV Eevees and the Eevee candy I get from the transfer of lesser Eevees (in wait for new Eevee evolutions).

Scyther, Ditto, a family of high IV Horsea and 1400+ candies, ~3 of each pkmn that will evolve in future gen.

I personally hold onto all my rares (such as Snorlax, Lickitung, Ditto, Porygon and perfect IV mons) because they're nice trophies to have and transfering them just wouldn't be worth the one miserable candy.

Santa Pikachus, on the other hand, weren't rare at all, everyone has them and 21 are way too much to keep. I just kept one with Thundershock + Thunder for training, and evolved one with good IV into a Santa Raichu.

If you're planning this for trading, however, don't get your hopes up. We might have to wait a looong time for it.

Been holding onto rares in case of double candy event. Currently Snorlax, Dratini, Horsea, Bulbasaur, Abra, Poliwag, Rhyhorn, Scyther are some. Also holding onto high IV versions of the ones that evolve in Gen 2. My storage is typically around 900/1000 lol.

I'm holding on to about 10 Santa Pikachu, Ditto I keep highest CP and highest IV; I keep all high IV mons that get evolutions in Gen 2 (Seadra, Golbat, Eevee, Slowbro, Poliwhirl etc etc.) and some decent Mr Mime for trading.

I wish I had to worry about holding on to Licktung and Porygon, I only ever got two of each.

raichu's are great prestigers. santa raichus? even better!

I just keep the highest cp and highest IV ditto.

the other two, keep unless super low cp.