Best and most accurate can and will vary between guns, even between guns of the same model. The only way to really tell is buy a box and hit the range.

As far as the type of bullets, you need to give us some info regarding the intended use of the gun.

270 Win ammo can be had in bullet weights from around 100gr all the way up to about 160gr. 130 and 150 being the most common.

For up to deer sized game, any of the 130gr bullets will do the job.

Larger or tougher game will have you leaning toward the heavier 150gr variety.

Regardless of which bullets you choose, when you find the bullet that shoots most accurately in your gun, buy a bunch of them.