You could start with Eley Black Box, then try Lapua's second-best, then RWS 50.
I wouldn't waste money by getting Eley Tenex or Lapua's best until you find out which brand ammo the rifle likes best. I haven't had good luck with RWS cheaper ammos, but that doesn't mean you wouldn't have good luck with them.
Most of the lower grades in the big three are lots that just didn't measure up to factory standards for the best. That doesn't mean it won't shoot just as well in YOUR rifle. Many benchrest matches are won with second-line ammo.
For general shooting, I've found CCI Standard Velocity to be very consistent and some lots capable of 1/2" groups at 50 yards.