OK,
Probably the _best_ thing to play would be a marshal 4/???? X (??? is probably a paladin, maybe a sorc or bard). Providing +6-8 to overcome SR will make you popular. Shifting that to +6-8 damage when flanking will also be nice...
But what I like best for this party is:
Human vow of poverty spirit shaman. Lots and lots of flexibility (esp with the bonus feats from VoP). Lots of roleplaying potential. Lots of spells. Your party has direct damage covered, so not having that is no great loss. And against spirits you will be very helpful indeed.
Because I've got time on my hands and this is fun:
One obvious change would be to go with a level of battle dancer rather than monk and boos CHR rather than Wis.
Monk 1/Spirit shaman 11
STR 10
INT 10
WIS 16 +3 (+4)
DEX 14
CON 14
CHR 14 (+2)
hitpoints: 81 (depending on method)
AC 29 (+2 dex, +6 wis, +8 armor, +2 deflect, +1 natural)
AL: LG
Feats:
Sacred Vow (Human)
Vow of Poverty (1st)
Improved Grapple (monk)
Nymph's kiss (VoP 2nd)
Spell focus: Conjuration (3rd)
Intuitive Attack (VoP 4rd)
Augment summoning (6th)
Nimbus of light (VoP 6th)
Holy Radiance (VoP 8th)
Spontanious healer (9th)
Touch of golden ice (VoP 10th)
Stigmata (VoP 12th)
???? (12th)
Skills:
Variable, but max out diplomacy. Between the CHR and misc bonuses from the VoP feats (+6 total?), you will be a good face.
In hand-to-hand you are pretty weak, but not horrible. Between the monk abilites, the intuitive strike, the VoP bonuses and the touch of golden ice you will be ok. Being grappled will suck (though improved grapple will help a bit) but spirit form is an SU ability so once a day you should generally be able to get out of it, but it will slow you a fair bit. Also, touch of golden ice will annoy most evil grapplers.
With Spon. healer you free up some very useful selections. And in combat you can more-or-less raise dead 1/week. Handy for your party.
With Guide magic a lot of fairly worthless spells become huge.
And most importantly, I think the character would be a joy to play. You don't need to worry about stuff, you will have a very different world-view than everyone else, and you can be all wacky mystical (while still LG).