What type of buff offers the most damage NP wise

I assume you mean from ce or support choices. Depends on the servants in a lot of cases. Salter has a buster and attack up skills, so would benefit more with an np up. Drake on the other hand has an np and attak up skills so benefits more with buster up. The same type of buffs are additive, while different types of buffs are multiplicative in terms of damage.

Different buffs stack multiplicatively.

EX: Caesar has NP up and attack up. Give him a Quick up CE/boost from someone else (Alexander, Atalanta, Helena, Scat Lancer), and he is going to cause more dmg with this than just adding more attack/NP ups.

Combinations of multiple types of buffs, along with defense down for enemies would get you the highest damage numbers. For example, a 40% attack boost is inferior to a 20% attack boost in combination with a 20% card type boost, as they multiply, getting you 1.44 x your damage, compared to the flat 1.4. And this multiplication continues on for each type of damage boost. But if you're just looking to find what is the single biggest buff you can get, it would probably depend on your attackers.

Servants with card specific targetable buffs have some of the highest multipliers, like Tamamo, Scathach and Merlin with a 50% Arts, Quick, and Buster buff respectively at max. There are also decent options for party wide card buffs, but their effectiveness varies heavily based on your own composition.

Attack buffs work regardless of the card type, making them a far more consistent option, but their multipliers vary heavily, with the largest buffs usually being self-buffs, or two-part buffs that require specific traits, limiting the pool. Out of a single skill, you can get something from 20-40%, or 50% in the future from Nero Caster.

NP Damage buffs are usually a small cherry on top in terms of damage increase if they come from skills, as their multipliers are relatively low, and they typically cannot be used repeatedly. However, CEs like the Black Grail, Holy Grail, and Angel's Song offer a good chunk of damage, with the Black Grail itself being one of the largest multipliers at 60, or 80% at MLB.

Technically, yes, those 3 types of buffs do influence damage NP wise. However, what provides the most damage in the long run will vary from servant to servant on what CEs they have equipped, what their skills are, even said servant's class skills.
Card specific CEs that increase Buster/Quick/Arts tend to be the most common types of CEs when it comes to increasing damage output in a multiplicative sense. CEs that increase ATK like "Golden Sumo" for example are generally the more sought after CEs for damage output since it's not necessarily card specific for Buster/Quick/Arts or/and NP damage. NP damage up CEs are kind of the middle ground when it comes to rarity. However, if you're lucky enough to get a CE that can buff both NP damage with a card specific buff for Buster/Quick/Arts or an ATK up buff, consider using those if you have any (i.e. Kaleid Ruby, Kaleid Saphire, Hero Elly's Adventure, etc.)
In short, CEs, skills and class skills that buff Buster/Quick/Arts will buff said type of card(s), regardless if it's a regular attack or an NP. CEs, skills and class skills that buff ATK up will buff damage output in a general sense, regardless if it's a regular attack or an NP. Lastly, CEs, skills and class skills that buff NP damage is pretty self-explanatory; it only buffs the damage output of said NP, regardless if it's a Buster/Quick/Arts card.
The best advice I can give if you want to increase as much NP damage as possible is utilizing those 3 types of buffs that you mentioned are by using CEs, skills and class skills to buff said criteria.
I apologize for this answer being very long, but hopefully this will get my point across