Quests vary so much in an RPG, from the simple "fetch quest" to the more complicated kind that affect an individual or group of people. I never play as an "evil" character or one that's interested in only profit or power. I play characters (or party of characters) that try to help people, especially those oppressed or impoverished, I've even turned down quest rewards because doing so would help someone else. It's kind of my own weird RPG persona, a game moral code, that I've maintained over the years. It's not always successful, and there are many variables in a complex quest, especially when there is no "good or evil choice", just a choice you have to make.
Fetch quests like, "I need 10 sheep pelts to finish this coat", or "I need the Amulet of Bigusdicus from the Tomb of Small Hands"; quests like those I always accept, but they tend to be on the backburner and those I'll do when I'm in the area.
Quests that take priority are those where someone, a group of people are dying or in danger. Some are immediate, some have a time limit, and some are ongoing; but all those types of quests take priority. There're many variables of course, and the best of those don't always have simple solutions.
The attitude of the quest giver doesn't really affect my decisions in most cases. I've completed quests for a nasty or rude person where I was able to convince him/her to my way of thinking or gain information that I wouldn't normally have gotten. Variables, choices, consequences are all part of a good quest.
I've also turned down many quests, not because of the quest givers persona, but because of his/her intent wasn't something I could live with, especially when it comes to slavers. I don't work for slavers, nor will I tolerate their existence, even if my character could personally profit from it.
Take for example a quest I got recently in Wasteland 3. I returned to my homebase and was confronted by a group of slavers that wanted me to retrieve a young woman (Halima? not really sure on the name) who was their "property" and had escaped. They offered the codes to get into the locked vault in my base (which is not accessible with skills). In a lengthy conversation the option became available to attack, and I took it because I don't deal with slavers. Maybe there was a better way, and maybe I lost my chance at the vault, but I felt good for destroying them.