For me, it was finding a goddamn Moon Stone for the second-last Moon Stone-evolver I needed for my Pokédex... then realising it was the second-last, and just about screaming.
Elesa's gym was evil... I got paralysed practically every battle. I have this thing about keeping all my team at the same level, so I didn't just sweep with Palpitoad (not that I could. Blasted Emolga.)
On White, though, most of the game was a breeze... except when I was using Deino. (I tend to run a starter+3 dragons team on my second run of any region.) Hustle sucks. Funny thing, though: I had mad luck with Dragon Rush.
N and Ghetsis were pushovers both times (though I grinded crazily on White to get Zweilous before the Elite Four.) I didn't even use Reshiram/Zekrom - they were sent straight to the box. Hydreigon is no worry when you have a Haxorus with 2 DDs under its belt. (Cofagrigus: I can't kill you oh noes. Haxorus: Dragon Dance! Dragon Dance! Sweep!)
Lastly: VICTINI on White! That thing just wouldn't stay in a Ball! Until I got it in a Heal Ball (which I also got Zekrom in... weird, huh?)