First, second and third choices, bard. Fourth choice, probably cleric.
Bards have:
Good spell selection, including
Movement spells (dimension door, shadow walk)
Information-gathering spells (clairvoyance/audience, scry, legend lore greater scry)
Infiltration (invisibility, silence, sculpt sound, greater invisibility)
Misdirection (the image spells, veil)
Communication (comprehend languages, tongues)
Healing
as well as a range of 'save-or-pretty-much-take-you-out-of-the-combat' spells, starting with glitterdust and confusion.... and for the odd spell which isn't accessible to bards that might be handy to have (restoration, commune, plane shift, discern location etc,) they can UMD a scroll!
Excellent skill selection, including
Social skills (Diplomacy, Sense Motive, Bluff, Gather Information, K(N&R))
Knowledge skills
Infiltration/scouting skills (Hide, Move Silently, Listen)
Bardic Knowledge ('nuff said)
Mediocre combat skills - a little better using songs and spells to buff - but having said that, no class other than fighter/ranger/paladin is likely to be able to take on directly in combat (rather than via spells,) opponents of comparable CR. Possibly a wildshaped druid, but I haven't had enough experience with them to know. Losing armour, weapons, and other equipment - and the low AC - seem to me to be big disadvantages.
What other class can Gather Information to find out what's going on in the first place, use Diplomacy, Sense Motive and possibly Bluff to help find out what sort of people she's dealing with and whether there are hidden agendas, magically scout out the target area and use Bardic and/or mundane knowledge to go in with more information than any other class and get past any left over hazards by movement spells, misdirection spells or stealth skills; confuse, enthrall, suggest, charm, charm or bluff your way past enemies whom you don't avoid, and at a last resort still have some combat spells, fighting ability and healing for afterward?