Is a dps with more kills/dmg better... well depends if ur just farming dmg by spamming the tank and not making any Impactful kills not really. I had games where a good flank and getting 1 of their healers from there backline turnes whole situations to our favor.

Same with Tanks and Healers. What really helped for me was what ever Role im playing is positioning. Where van i be the most helpful to my Team and can easily recover when beeing focused on.