In judging the most 'moral' character you of course run into the philosophical problem of whether there is such a thing as a universal morality to use as a benchmark, or whether all such judgements are essentially culturally relative. Now, I tend to think that it's useful to preserve a notion of cultural relativism when discussing real life morality, because in that case its useful for discussing how 'evil' regimes and acts like genocide can come to be constructed by particular societies as perfectly rational and 'moral' acts (such as in nazi germany for instance). In other words without remembering that cultural notions of 'morality' can be twisted to justify horrific behaviour we leave ourselves vulnerable to repeating those behaviours while thinking that 'ours' is the most universal and basic morality. I don't know how applicable this is to describing fictional characters and societies however, as in this case its all made up anyway and the setting kind of comes with an in-built morality scale specific to our own culture - the bad guys are bad and the good guys are good, at least most of the time.
It might perhaps be useful in this case to think of a matrix like d&d character alignments; with an axis of 'lawful' (respect for society's rules) to 'chaotic' (rebelliousness and individualism), and an axis of 'good' (altruism and respect for life) to 'evil' (selfishness and no respect for life). I'd argue Odo is probably the most lawful character of DS9, and probably a good or at least neutral alignment. Someone like Rom, Jake or Leeta is probably more 'good' than Odo but not necessarily more lawful. The female changeling on the other hand is textbook lawful evil, believing firmly in the rules of her society but also embracing the selfish 'superiority' of that society over all other life. Sisko or even Garak on the other hand are probably examples of chaotic good, willing to break all the rules pretty much but generally for altruistic reasons.
