Can anyone please explain the difference between words: moraliser and moralist? Which one has got more positive meaning? Which one would be more appropriate, for example, in such a sentence: Charles Dickens was a great moraliser/moralist.
Both have negative connotations:
mor‧al‧ize also moralise
British English [intransitive]
to tell other people your ideas about right and wrong behaviour, especially when they have not asked for your opinion [= preach]:
And I am sure the above isn't only BrE, but also AmE.
Now about the other. See 3:
1 : one who leads a moral life
2 : a teacher or student of morals : a thinker or writer concerned with moral principles and problems
3 : one who is concerned to regulate the morals of others
Merriam-Webster Unabridged Dictionary
However, I feel that the first has more of this negative connotation.