Speaking of TF, is there any reason to use Diddy? Dixie's ability seems better in every way. Also do they all have the same properties like running speed and jump length?
By himself he's the fastest Kong, but on DK's back he's overshadowed by Dixie, especially if she's on his back. The double jump is simply better than Diddy's hover and has a slight edge over Cranky's cane hop, and if you're facing a tough time Dixie should be your first choice, whether it's hard mode or not, ESPECIALLY underwater.
Cranky's the slowest kong on foot (key words), but if you get into a good rhythm of cane bouncing you can rip through the stage faster than Diddy by himself in many cases; this is great because that cane bouncing speed works whether Cranky's alone or on DK's back, whereas Diddy's speed only kicks in when he's by himself or underwater--in which case he's a speed demon regardless of riding DK or going solo.
Basically Diddy's the fastest, followed by Dixie then Cranky, but there are ways around it sometimes. Dixie might end up being the go-to kong, but I recommend switching up between them, especially in mid-level as they provide different ways to tackle a challenge, and some spots require specific kongs.
I REALLY wish DK had a special thing of his own...even that soundwave clap from Jungle Beat would go a long way, but he's annoyingly normal in this game. It's like if a hero without powers had super sidekicks with crazy gadgets. Playing solo with him brings this issue to the forefront, but I guess it's a test of skill.