Zodd and SK were both on the list for me at some point, but after consideration I realized that Zodd, despite being totally awesome and badass, is not a particularly "deep" or "round" character(his character/story and motivation can be outlined in 3 to 4 sentences, and he's yet to show any kind of moral conflict. Siding with Guts against Ganishka was great, but still I don't see him ever doing a full-on faction-change), and SK would have only been my pick because I WANT to see him in the story the most. Meaning that when SK shows up, pivotal plot movement is taking place. Almost every word that comes out of SK's mouth... er... helmet? ...tells us something we didn't know. But that's not a reflection of his character. He's awesome and I'd trade ten Guts scenes for one SK scene any day, but He's not my favorite character. Valiant knights out for justice are cool, but not uncommon. This could EASILY change with concrete revelations about who he was in the past (History between him and Void/Slan for example.)
Just to quickly comment on what you said (not disputing your choices), I think you're being a bit unfair with Zodd and SK here. Although Zodd is an apostle and so is by definition going to fit in certain categories, he's been given some surprising depth at times. It's remained light so far, but the foundations for later developments are definitely there. Be it his past and his relationship to the Skull Knight, his attitude towards other apostles, the God Hand and associated events, or his actions towards Guts, there's more to him than meets the eye. And so I believe his assumed status as an important character will be justified with more than enough depth in the end, even without having to change factions.
Same thing for the Skull Knight, I think you're selling him a bit short by reducing him to a "valiant knight out for justice". Even without having definite answers yet it's pretty clear that he's got pivotal background information waiting to be revealed.
Griffith was a definite choice at first too, but lately its hard to tell exactly who Griffith is. He has almost no personality at this point, and unlike in the early volumes, now he has no passion/motivation that we're certainly aware of. I watch him now and its like he's just going through the motions. He hasn't appeared to be 'passionate' in one scene [...] I haven't seen that side of him in a long long time now. 
While I totally understand how you feel, the reason is that "Griffith", as in the human Griffith that was shown to us during the Golden Age arc, doesn't exist anymore. He was reborn as Femto during the Occultation ceremony, and although Femto has since then acquired a body of flesh that conveniently bears the guise of his old self, he is definitely not the man he once was. His goals, motives, feelings, etc. are all different now, and the reason we get (almost) no insight into his mind is so that it remains alien to us.
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