Mercs wont be available until SoF or SoD unless I am mistaken but an actual clr char couldnt hurt the line up.
Tanks
I am a fan of the SK class when it comes to tanking. Best tank in game imo for grouping content and later on can even handle some raid targets tho those are limited. They are more reliant on AA than war's to do the job tho but if you get those AA the number of heals they require from your healer is much much less. ToC crits for 1.4k now in OoW and that is innate from melee, direct taps double crit for over 2k, shroud double crits for 1.6k, plus whatever innate procs you may have on weap or augs on that weapon.
War innate 5% mitigation bonus is pretty nice and with their disc can do better job of handling mob dmg than any other tanks. However, they are totally reliant on others for buffs beyond the available clickies or potions. I box but still prefer the SK over war due to his limitations and the SK's ability to fill multiple roles beyond that of the tank.
Pallies I've heard a great deal about when it comes to solo'ing (undead) and ability to stun for agro and reduce dmg taken. Heals tho require changing targets to cast, can be interrupted, and stuns do have a chance of being resisted if mobs arent innately immune. (go go Shadowknight taps innate, activated, and buffed so no need to change targets but have -200 or higher resist mod built into)
Healer
Druid I've played with and while not bad in the boxing scenario on progression they play one of two roles... that of the healer (poorly) or dps. Great plvlers and decent dps currently but while I do have multiple of the class they are limited to strictly plvling, foraging, or transport. Don't get HoT's until much later in lvl and while skin is nice for casters and dps is decent I'd much prefer pure dps classes.
Shm hell ya. Shm is must have in pretty much all of my groups. As a char whore I have multiples of this class as well for any group line up I choose. Heals, HoT's, slows, stat debuffs, and stat buffs you just cant go wrong having one in group. Hell Epic will even increase burn dps for melee's or caster pets with pet affinity.
Clr - mercs really did kinda kill new comers to the class imo for the most part. Since mercs not available tho the best way to get that clr loving such as Virtue or Conviction is to have one of your own. On progression servers since the lvl is so low the paci line can actually play a role long forgotten by those on the regular servers once again.
Puller
SK plays this role as well as any other... unless you want him tanking and shit incoming before the last mob is dead.
In that case I believe Fade is currently available to bards and monks usual techniques are always available.
Bards caster dps increasing songs are not available just yet I dont think but could be mistaken.... only played as a box and not until SoF when I think those lines first went in so may wanna double check Alla's or with higher lvl online bard to confirm that.
Mnks skills are always available and they can back up as dps should it be needed. I dont personally trust moveutils to handle the control of the class so positioning or pulling would have to be done manually but there may be bots out there to handle that if you are ok with them.
Enc always nice to have around for crack for your casters and the nice boost their haste gives to agi, dex. Add in their tash, mez, and other crowd control techniques and they also a nice to have class in box set even if you already have bard/shm for mez and slows.
DPS
Mnk see above...
Rog I have a buddy who always has one he just loves the BS dps and dmg they can do but personally I dont care for since that dps is almost entirely dependent on having the mobs back available to you and no ping ponging of agro. Good class for single account player but not so much boxing.
Zerk is my class of choice for pure melee dps. They are highly reliant on AA but with those AA and various disc can burn for over 3m or repeatedly in the normal respawn time for group content named if you camping multiple named from the same location. Being chain class they can also tank for a time if your tank does happen to die and they dont rely on the mobs back to continue to dps when it happens.
Nec I loved as a solo'ing class and dont think any others can match them vs non summoning mobs. Group content limits them though due to their sustained dps requiring the build up of multiple dots and mobs usually dying in under 1m in group content. Raids sure, solo'ing sure... group content they just better off solo'ing or grouping with other caster dps for quick kills.
Mage is another class I absolutely love. Prop's go out to Dev for his Mage Bot here! That is what drew me to the class but further use of it outside of that is what made me a true believer. Tank dies pets can and have for me finished off fights in UF and HoTT content before in end tier group content. Progression server I have repeatedly pulled multiple targets at a time knowing that the mage pet could offtank nearly as well as my SK. Hell during LDoN content I always pulled 2 or 3 intentionally with the idea of my mage pet offtanking one of the mobs and he did so 99.9999999% of the time with no problems. Add in pet dmg to mages normal nuke dps and at progression lvl you have two classes that equal each other easily. The gap does widen at later lvls but not so significantly that I would choose wiz over mage. Pets ability to back up tank for short periods of time is just too much to ignore as a boxer.
Wiz yup burns, evac, transport high value skills but if dps slots are limited gimmie mage over wiz. Multiple slots available tho sure wiz burn rate is better and higher dps will shorten fights to point your tank might not die so I wouldnt rule out the wiz.
Only 3 (or 4 box if buddy not online) but if I did 6 box on progression I'd prolly go with the following line up:
Sk, Shm, Clr, Mage, Mage, Wiz.... with enc alt on shm account up to 60 for KEI.
Wiz over Zerk only because I dont trust moveutils to position w/o occasional circling which is obvious hacking behavior so relying on manual positioning for zerk vs using a bot to assist and cast as needed hands free. If you find a hands free way of running pure melee dps vs caster I'd use zerk over wiz every day(no resist checks to a blow to the head vs spell).
Multiple mages cause Dev's mage bot is the shit and multiple air pets constantly stunning mobs = minimal dmg. Seriously that pet stun fires every 12-18sec and last at least 3-6sec. Add in the dmg done with normal melee hits during stuns and you got mobs that dont move to far with multiple air pets on them even when flee'ing so who needs snare if you chose war over sk.
That line up you may not have incoming before the current is dead but even if SK snare/FD pulls mage can tag and pet tank until agro is clear until you get the HA aa on SK in later expansions. Also if you pull smart you dont have to split everything just central add targets so only first pull or three requires FD splitting rest are all single or douple pulls easily handled.