Generally I find that Space Marine HQs are less points-efficient than other units, therefore I find spending more points on them than necessary is a waste. So HQ choice -that's my viewpoint- should follow necessities. And since special characters are usually more expensive (it doesn't help that they're better) than generic HQs I tend not to take them.
I use my generic bike captain most of the time because I need him in order to be able to take bikes as troops. Sometimes I take a master of the forge if I need a conversion beam or want to make a more-than-three-dreadnoughts list, but he's no special character either.
Cassius, for one, is a very cheap and a very effective special character. I used him and I can only recommend him. I've tried Pedro Kantor and he is a perfect choice for sternguard-heavy armies that most probably are unable to afford many troops. Shrike's infiltrate is very nice too, but the army needs to be built around that. Same is true for Vulcan. Even Khan I could imagine to have his place, but the others are just too expensive for my taste. Lysander is very tough and can really bolster a gunline, but those army builds are not so effective in general.
So it comes down to this: I don't recommend using special characters. If you do, then probably Cassius is the best choice. Or Pedro, Shrike, Vulcan, and maybe even Khan - each if your army is built around their special abilities...