I stick with samples for kicks, and when I find a good kick, I don't let go of it.
I like to load my kicks up into CoolEdit 2000 and compress them there. I can see exactly what the compression does to the waveform and have gotten used to which types of compression fatten up the kick sound. With the right settings, you can compress the kick and use the makeup gain in one setting at the same time.
I have noticed that kicks that linger a long time sometimes seem to lack impact. Often a kick that pounds, is one that goes quickly to full amplitude, holds for just a while, long enough to be heard and felt, and then quickly fades out--sometimes even an abrupt fade out.
Because of this, sometimes I go back and edit my kicks to actually be shorter in length. This makes them tighter for lack of a better term.
I also trim the starts of kicks to make sure that they start right up to full amplitude without any kind of delay. I'll do a tiny fade in to keep them from clicking, but sometimes the click sounds good.
Typically, I have found that synth kicks are a lot more powerful than most (not all) acoustic kicks.
I've also found that a lot of rap kit kicks mix hihats or use low-quality sampled acoustic kicks. This is naff. I like to use kicks that are purely synth kick, or high quality acoustic kick (if it's good enough). If you need to have a hihat with your kick, you can mix it in later, during composition. Having a hihat in a kick usually makes it weaker if you ask me. But I understand the concept...
A good kick won't be 100% bass, but will have some treble coiled up at it's beginning. This helps it to cut through the mix and be audible, especially on speakers and playback systems that don't have much bass.
That's my 2 cents. But wait, there's more
....
That's just how I treat raw samples. When I mix, I typically boost kicks with a wide parametric hump centered around 140 Hz. Then I'll often add a shelving EQ boost from about 120 Hz on down. Surprisingly, on some kicks, to boost the click/attack of it, I'll shelving EQ boost the treble frequencies, say from 1kHz on up. It really depends upon the sound file and what it sounds like. There's no exact frequencies used every time.