Buffing in core is really about long-duration spells you cast at the start of the day and a handful of in-combat buffs. You will probably end up using buffs that don't multitarget just because, in core, there aren't enough buffs that do, and you will need to give people specific abilities not available as multitarget buffs.
The most important long-duration buff is greater magic weapon. The only other long-duration buff that helps offense is heroism. Air walk is good in the long run since it gives your party, as well as you, flight. Waterwalk is more situational but in the same vein. Darkvision can be important if you're dungeon crawling with races that don't get darkvision intrinsically. Barkskin, magic vestment, delay/neutralize poison, and heroes' feast are good defensive buffs but as always are a lower priority than offense or utility abilities.
As for in-combat buffs, haste is the best buff for weapon users in the core, without question. Rage, good hope, prayer, and mass enlarge person are also good depending on your party and what bonus types they're already getting, though only mass enlarge person can really swing a combat. Bardic music isn't that strong in the core, but neither are the spells, so later on inspire courage or inspire greatness can be a reasonable choice instead of one of the multitarget buff spells. By the time you get the mass versions of bull's strength and cat's grace, they're obsolete because everyone should have items that give bonuses as large already. Align weapon can be very important but there's no mass version in the core.
If you have any sneak attackers, battlefield control like grease and black tentacles and spells that inflict some status conditions like blindness/deafness can set them up for sneak attack.
There's not much more to buffing in core beyond picking an appropriate class, bard, cleric, or wizard, not losing caster levels, and picking the right spells. Only archmage can really help you buff. Loremaster doesn't help your core competency but might be better than taking more levels of whatever your base class is. Feats-wise, Craft Wondrous Item is always good if your DM lets it put you ahead on gold, and you need Extend Spell to make your long-duration buffs last long enough. You might want Spell Focus or Spell Penetration if you're debuffing or casting battlefield control. Beyond that, no feats really help you, so you might as well grab things like Improved Initiative.
