Gundam Seed & Seed Destiny
Code Geass - R1 & R2
I recently just started doing dru- anime so I haven't watched that much for about a year. But just an interesting anecdote; I ended up doing a hardcore marathon on both Seed Destiny and Code Geass, finished them both in about 4-5 days...
I felt miserable after that, though.
Mainly because I was deprived of sleep, and partly because I had nothing to watch afterwards.
Wait I thought I read anime... Not episode specifically.
Well in that case it'd have to be Code Geass R2 ep 25. The last one, and a great twist it was.