Without doubt, stumbling into a wild Dragonite without looking at nearby (before weather boost and TM). It was lvl 30, 15 atk, 15 def and 8 st with dragon tail/outrage.
Out of my 140 000 catches I have only got 9 wild Dragonite (actually 3 of them just occasionally spawned in front of me). The chance of getting lvl 30, 15 in atk and such good IV and the best out of six move sets, in addition, is very small. I tried to calculate it and ended up way above 1 in 100 million.