The episode in season 7 (i think) where marshall and Barney are fighting and Robin and Lily go through all the different alcoholic beverages that they can give them based on how they changes there moods and behaviours to make them make up and be friends again.
Tick Tick Tick (season 7 episode 10) where Robin and Barney cheat on their significant others with each other is my favorite because no matter what I will always end up crying at the end. I love Barney so much I just want to hold him at the end of this episode and basically all through out season 7 I end up crying.
I like the one "Come On" It's the finale of season one. Ted makes it rain so Robin won't go on the company camping trip. The end is such a great scene because its the moment where they finally get together.