I'd argue the best Marshal only has a few levels of marshal. Frankly, Marshal is pretty weak as you get past 4th level. Throw in some bard and/or some dragon shaman and you've got a hugely powerful buffer. Silly powerful in fact.
Say Marshal 4, Bard 8. CHR 18+2 (levels) +6 (item)=26. On your first action sing and cast insperational boost. At that point you could be giving all allies:
+4 attack, +3 damage (+11 if flanking)
-- or --
+4 attack, +3 damage, +8 to overcome spell resistance.
And you can grant a move action 1/day to the whole party. Mightly handy in the "big fight".
If you are in Eberron the attack and damage goes up by +1 due to Eberron specific feat.
Crazy powerful. With Dragon shaman it gets only a bit more powerful, but a LOT more flexible.