I don't really remember if all of those fillers are canon, but I know that Kanda's Witch Hunt arc is canon based on novels, story with hospital invastigation is canon, and even story with Gladiator and elixir of life. Plus to that some flashbacks of dead characters are canon so...yeah most of those fillers are canon.
Soin this case I can say that when you first time watching DGM or reading better stick with manga or skip all fillers. But you should watch at least some of them, case some of them, especially Kanda's arc and Hospital
Hmm my top 10 would be:
1. Kanda - despite the fact, that I dislike characters like Zoro, who are arrogant and etc, Kanda is different. Yes in the begining he looked like another Zoro, but then I grew on him. He was showed as good real leader who can take responsibility for own questions and for his teammates. This is why Kanda became a General. So yes, number one character for me.
2. Allen Walker - I was thinking that he should be on first place. Now I will say he is on second place, but this can change soon enough. Sad clown whose smile hide his tears. He and Kanda are best charters for me in this manga and anime;
3. Road Kamelot - ok, maybe this is just me, but I grew a lot on her in this flashback. And now I can say that when I reread first chapters with her, everything that she said and what she did got hidden context. So litteraly she became character that I'm really interesting to learn and see even more;
4. Earl/Mana
5. Komui - best brother is here;
6. Marian Cross
7. LeenaLee
8. Neah;
9. Krory
10. Lavi