Let's see here. In DKC1 I find myself using both of them about evenly. I really liked Diddy a lot more back then than I do and maybe DK a little less than I do now. But DKC1 I really played it during my casual gameplay days and can't really describe who I used more. I'd probably use them both just about evenly now.
In DKC 2 however I find myself using Dixie. I'm always filled with disappoint when I lose her and I'm stuck with Diddy Kong till the next tag barrel.
I really enjoy soaring just about endlessly throughout the level. A really broken mechanic if you ask me. Same applies for DKC 3. I really dislike sluggish characters so Kiddy kong was not my cup of tea. I definitely used Dixie for more beyond him whenever I got the chance, however I would resort to Kiddy when those exclusive moments arise.
Donkey Kong Country Returns.. Well. I pretty much played throughout the game alone but when the chance was given to me (one could identify this as never) I would use Diddy Kong. However if I were to go back and play the game again in two-players I'd probably prefer DK. I would usually consider myself the more advanced player at this day of age and therefore offer the break to whomever i'm playing with.
Tropical Freeze.. Man oh man. I love how you're given the choice. CRANKY ALL THE WAY! I love the way he plays, and he's probably my most favorite playable character in the DK universe. I was the most excited to finally have him playable in the mainline games during 2013's VGX (the only thing actually worthwhile announced there I might add.) However I would of course resort to Dixie when I must use her for secrets. Even in two players I would insist on playing as Cranky unlike the previous installments.
As for K. Rool victory theme I was just think of an extended version of Gangplank's Galleon first few notes. Probably similar to brawl's remix from outro transitioning to intro. I'm sure there are better options to choose from.