This will most likely be a heavily debated thread due to the fact that everyone believes there job is the best in its class. I am throwing this out there to get some opinions other than my own about what is the best party set-up.
I have been in many parties and I have found the ones that have worked best for lvl 40+, given you have players that know thier jobs, and have good equipment on would be:
1. Whitemage - Your direct Healer, all the best "Ra" spells. Yes a RDM can replace a WHM lvl 40+, but for an RDM to be at thier best, they cannot be the main healer. A SMN can also replace a WHM, but they do not have all the best "Ra" spells. @nd to WHM in healing for a party though would definetly be SMN if you cannot find an available WHM. SMN's also have very nice effects from thier Avatars to help a party be at its best.
2. Red Mage - DeBuff, Convert, Refresh, Melee, Back up Healer. RDM's are the Jack of all trades and one of the most difficult jobs in the game. I hear how BRD's are just as good as RDM's, and given I dont know much about the job, I cannot say that they aren't, but I do know BRD's do not make good Back up healers and lack the job ability of convert.
3. Black mage - Debuffs,Escape,Warp2, a cure here and there when much needed, and they deal the most damage of any job. MB Freeze, need I say more?. There is no other job comparable to what a BLM can do for a party.
4. Paladin - Your much needed Tank, no party is complete without a Tank and PLD is the best at it. Keeping the Hate with Flash and Curing themselves and good PLD's all help cure other when needed. Theres no such thing as a party with too many back up healers. I also must include that a Very Good NIN/WAR can compete with a PLD at keeping the hate and even though they dont have MP to back up heal they save so much MP from mages by Blink Tanking that they more than make up for it. I have only seen a couple NIN's that compare to a PLD though, for the most part your avereage NIN doesnt have the skill or the gil to keep the hate like a PLD.
5. Dark Knight - Asorb spells, MB's, The consistent Damage output they provide from their Great Job Abilities. A Dark Knight to be good must, for one know thier job well, and for 2 must have all the plus acc they possibly can have. This is a low acc job with a large weapon delay, and if your missing frequently it will hurt your party in a big way. SAM,DRG,MNK,WAR these jobs are better than a bad DRK in a party, but cannot match a DRK that has all the equipment needed and one who knows their job. This decision will proably be the most debated, but all those other jobs lack the MP abilities of a DRK.
6. Ranger - The most efficent puller in the game due to thier massive damage output from thier ranged attacks, and thier damage output through the fight. THF is also a very good alternative to a RNG. THF's can be very good pullers, and their SATA does crazy Damage to the mob, plus when your looking for help from a THF with TH2 and wonder why there arent enough, just remember that when you are forming your party, keep the THF's happy with invites and they will be more likely to help us out when we need them.
I have put myself out on the limb here, I know alot of people are going to be pissed and other will agree. I have not seen a thread like this in the forum yet so I decided to throw it out there. I would also be interested in hearing peoples ideas on the best sub's for these jobs seeing I chose not to post those as well. This is just my opinion on the optimal party. Please post your thoughts on this subject as well!