With that said there are group setups that I favor more than others. I think my favorite group setup is starts with Me of course well a KotBS at least. Then we go with another tank probably a IB. Then the healers are a RP and a AM. For dps my favorite to group is a WH and a Slayer. Here are my reasons.
A KotBS because of our auras. We are the best group tank in the whole game with our auras running. I can make enemies in the area 10% more likely to be crit, while increase the healing doing to the group by 15%. We support the whole group with our utility. Plus with this single target group AoE stagger is king.
An IB because as a tank they have the best burst damage out of anyone. They can also survive a lot longer with what they can run. Plus the have gromil plating which is a very nice morale. They also have an AoE snare which is very powerful.
A RP because they have great group heals and good single target heals. They have the self rez ability which has saved our group many times before in different situations. There ruins are also very nice to have. They can also have very good survivability compared to an AM.
An AM because they are the best single target healer in the game imo. They have an ap drain which doesn't make them run out of ap very often. Plus they also have the viability to go dps if need be and they can be a force as a dps.
A WH and a Slayer because you can have the Slayer run up front and have all the enemies focus on him as the WH sneaks in the back to gank there healers. The Slayer has the best survivability of any dps and with guard is hard to take down. While the WH can just demolish any healer they are on if played right.
Well those are just my thoughts on it. I like running with every class but I just have the must fun running with those classes not matter who it is. If you feel like it post your favorite six man to run in.