everyone has one so whats yours for me it was:
1. dragonite- I got lucky with the elite four
2.quagsire- I never liked grass types
3. roselia and milotic - the amount of times my swampert was 1HKO by one of those was annoying and milotics were so annoying for me then
4.spirtomb - although I beat the champion first time in sinnoh spirtomb took down 4 pokemon until my lv55 newly evolved blissey took it down with one it
5. hydreigon- until I learnt that big bad things don't like bugs
so what about you?
edit I've just realised the title repeats to beat twice my bad.