Of all the things I like to write, I think the dialogue in action scenes is the most fun for me. I rarely plan dialogue in advance, so it's the most spontaneous writing I do, but oddly enough when there is a lot of action going on in the scene it's also the easiest. People under stress or duress say the damnedest things to each other, and I never get tired of composing it.
Action scene dialogue between secondary characters or a secondary character and the protagonist or antagonist is also one of the better story opportunities to showcase the support cast and avoid them becoming mere window-dressings or sidekicks for the protagonists. Here's one such scene from Shadowlight that I had a lot of fun with.
I wish love scene dialogue were as simple to write; I find that the most difficult to compose. I can handle describing the action, no problem, but cooking up the appropriate lines for the lovers to say to each other is always a challenge. Love scenes with no dialogue aren't as interesting, though, so I try to create and incorporate the right words.
One thing that helps me with writing love scene dialogue is to read some romantic poetry first. I don't quote the poems in love scenes, but they put me in the romantic mindset. Occasionally I'll write a love poem or letter from one character to the other (that helped a lot with writing the dialogue in the love scenes from Evermore.)
What are you favorite scenes to write, and what are your least favorite? Do you have anything you do to help with writing the scenes that are difficult for you? Let us know in comments.