I'm a monk and lately had a bit of time trying to find/form a party. When i am party leader and all other rolls are filled, here's what i like/look for.
Sam/*** - SC machines. I partied once in Sea Serpent Grotto with a Sam and he and I were able to pull off a SC every fight, sometimes two. Plus, we had the versatility to change SC's depending on the mob. Between the Sam and I, we had the potential for Fusion, Distortion (admittly the sam was on his own for that), and Impaction. Since we were doing Bones and Crabs, we rocked the house. My concerns with Sam is the growing trend that Sam/rng is the only way to go. I'm not saying its bad, but it leads me to feel that they aren't willing to try different things or make themselves available to switch to a more optimal setup. Plus, when a Sam/rng runs out of arrows (it happens from time to time), then their sub just got useless for what people sub it for.
Drk/*** - Awesome DD's, just hands down. I've met very few Drks that don't outdamage me in comparison of damage per hit. Yes, drks have a bit of an Acc. issue, but a good Drk has Jack-o-Lanterns at the ready. As I have made drk one of my secondary jobs, i understand the importance of those delicious wax coated pumpkins. My only concern is a Drk that doesn't grab his absorb spells when he is of lvl to use them. They help just as much as any other mage's debuffs, sometimes even helping the mages (Absorb-INT anyone?).
Drg/*** - I can't really comment on Drg's, since i don't party with them two much. Drg's biggest advantage is also what a lot of people see as their biggest weakness, their Wyvern. Yes, the wyvern's breath changes depending on sub, but since it isn't controlable (to the best of my limited knowledge), why only take a Drg/whm?
Mnk/*** - I love being a Monk. It fills me with great joy and happiness to beat things senseless with my fists. I do great DoT, have the hp to take a few hits if the tanks need a moment to cure/utsu/whatever (to all you paladins out there, you want hate? cast your highest lvl cure on an injured Monk. Garunteed to make the mob look at you and only you. (warning: all garuntees are not garunteed nor liable to be 100% true all the time.)), and we build TP real fast, second only to a RNG or SAM. Our only weakness' are enforced upon us by the rest of the players in the game when it comes to lvling. Most players prefer to lvl off Crabs (which hit for very little and don't have any truly heinous TP attacks) and most also want Distortion all the time. Mnks don't really ever do Distortion, sorry, but that's the truth. Our best SC is Fusion, because most of the WS of mnk are H2H and are non-elemental, so they close out Light and Dark SC's.
When it comes down to it for a party, here's what I would choose, presuming i'm playing as my Monk.
Sam - 1st
Drk - 2nd
Drg and Mnk - tied for 3rd.
Sam's are versatile for SC's, Drk's for Damage and absorbs, and Drg and Mnk because haven't played in too many parties that have either.