Consumables General Guide
In legion even pots have their own changes now. And the biggest one is that now we need use 2 different pots in a fight. The standard pure damage increase and one class - dependent (more below).
What you will need: Food, pots, flasks and runes(mandatory when we Raid). Depending your class and spec you ll need to buy/craft accordingly.
A rough estimate for pots and runes are: 2*Boss*1.5, for flasks depends on duration of the Raid, in our case 3 flasks we ll do the job just fine.
Baseline pot for damage dealing specs: http://www.wowhead.com/item=142117
Baseline pot for healing specs: http://www.wowhead.com/item=127835
Baseline pot for tanking specs: http://www.wowhead.com/item=142117 or http://www.wowhead.com/item=127845
Flasks: For flasks nothing really is changed for the past years so the guideline is pretty simple: you need intellect? go for intel flask, strength? go for str flask, etc...
Pot Changes:
On legion we have more role specific pots. You all going to be using the Prolonged Power, now days is best to use it on trash mobs, one but you will need one more to use throughout the raid.
For Ranged Dpss': the best single target pot is: http://www.wowhead.com/item=127843
For Melee Dpss': http://www.wowhead.com/item=127844
For Tanks: http://www.wowhead.com/item=127844 or http://www.wowhead.com/item=127845
For Healers: http://www.wowhead.com/item=127846
Summary:
Is best to have both kind of pots for certain scenarios, for example Nightmare Dragons encounter, u can prepot Prolonged Power and when it end u can use your role specific one so the cleave is easier.
Finally its up to you how to use em. Deadly grace and old War pots gives 2.5M pure damage on your target. That is why they are superior on single target. Although the Prolonged Power (since it doesn't increase your secondary stats but only int,str,agi,stam) its more powerfull if u want to cleave fast some targets and effectively raise your dps!
Thannks for taking the time to read through, if u have any further question feel free to ask me in game!