Cleric, druid, or wizard gets you the highest-level spells the soonest; since spells are the best way to buff anyone, these are the best classes for buffing. Arcanists are so absurdly overpowered that they can be a solid choice despite falling a level behind at 3rd; sorcerers with paragon surge can also do a decent job.
Clerics have the best non-spellcasting stats and features, between armor, HP, saves, and the options that Domains present. Druids are much more about self-buffing, and without exclusive access to their spell list, wizards seem a little lackluster.
On the other hand, the arcanist’s ability to switch up their (otherwise spontaneous) spells every day means that 50% of the time, you’ll have a massive improvement in your spellcasting.
For the purposes of buffing, arcanist and cleric are similar.
Because it deserves to be said: the bard’s probably the only class that has non-spellcasting buffing that’s worth considering. Unfortunately, Pathfinder nerfed the bard pretty significantly (partially by limiting how much music they can actually play, but mostly just by not reprinting various supplement options that the bard enjoyed in 3.5), and being 50% behind on spells is just not worth it unless you don’t plan on leaving low levels – though before 5th, the bard is comparable to a fullcaster and gets music, which is very nice for those levels.
Still, if you tend to optimize more than your table, bards are awesome, and can be very effective if built well. If the goal here is to avoid overshadowing your teammates, choosing the bard could be perfect. Their music is still a quite-solid buff, after all, and their spellcasting definitely doesn’t suck or anything.