Fire+Fire does do well for gaps, but the other combos are best for the reasons I posted.
Comparison:
Fire+Fire: Best for long horizontal levels, with little/none blocks/ledges in way, good for big gaps
FF vs. Rock+Ice:
Rock+Ice is superior for downhill, good for horizontal/some uphill (not as good as ice+cutter for uphill) has no delays whatsoever, bad for gaps
FF vs. Rock+Cutter: (assuming using bird form) Takes time to become the bird (frame dependent? if so then not, I seem to recall it not being dependant on how many times I changed but when I changed so most likely luck manipulation makes getting the bird first easy) , no delay in movement (with extra rolls when you land) good horizontal/vertical speed, can't go through slopes vertically (you have to go around them when going straight up) Limited number of flaps so not as much vertical height can be gained as with normal Kirby, but flaps are faster than normal Kirby's puffs
FF vs. Ice+Cutter:
little delay in starting/stopping, good for downhill/uphill, great jumping ability (hard to control but nets good results), potential gap crossing with perfectly timed jumps, only hurts enemies when you jump
FF vs. Rock+Spike:
constant speed slightly faster than normal walking but slower than fire, easy to kill enemies, no start delay, some stop delay, potential boss killer
FF vs. Fire+Bomb
Great vertical scaling combo, wonderful for killing enemies by creating a huge bomb radius, no losing horizontal speed, DO NOT use underwater, make sure the last blast is as close to a horizontal surface as possible for least amount of delay
Rock/Rock+Rock: Avoid these they provide less speed on downhills than ice+rock, and aren't the best boss killers either
I'd say to not rely on any one, but to use all three depending on the situation, you have to strategically change combos all the time to reach certain Crystals but if one level can quickly set up one of the non fire combos, then you shouldn't enter star world level one just to get Fire Fire because situationally, the places to use the other combos work best with said combos.
Boss Killers:
The best boss killing combos are:
Bomb+Cutter: The best in my opinion. using frame advance, it has the quickest damage output from a long range without making you fall down, so you can jump in midair and stay there throwing bombs. This combo is better than Rock+Spike. best place to use: Robot boss
Bomb: the SDA run had it right for the orca boss, I used Bomb+fire on the console when I first played but plain bomb is superior.
Light+Cutter: great speed, constant damage
Rock+Fire: used for one of the minibosses to get a crystal, but a machine gun volcano is never bad
Light+Light: you don't even have to aim, the radius is huge, the damage hits the boss on the first possible moment and kills all the stray enemies too.
Spike: pretty good, hits for consisten damage
Light+Spike: fast damage
Rock+Light: very fun, very hard to control, but you can get the lightning rock stuck and repeatedly damage the boss, or just get close and keep shooting out the rock over and over, because it is fast. But be warned, no floating while the rock is out.
also, if you only need the combo for a boss, you can take out the combo star and use it as the last hit
before each transition to the next part of the level, use a slide, making sure that with 1/4 of the slide left you hit the activation to walk to the next area so there is no delay
when/if anyone besides me starts this ( I am very tempted right now to start working on it after my DCK2 102%), plan in the run which combo to use at all given times in the entire run. For example, in Shiver star level 1 (snowland) you start with fire+fire, then pick up ice and cutter at the first enemies you can and use that all the way through level 2 (clouds) until you need to pick up light+spike for the mini boss. Look at each level beforehand to see if a speed combo is fast to get to- if not go ahead and use fire+fire if the level is not mostly uphill/vertical.
I think that's it. And like Bag of Magic Food said, some enemies do stuff when you hold them over your head- use em.
