What is the best multiple hit armor material?

Hands down, the best multiple hit material is Ultra High Molecular Weight Polyethylene (UHMWPE, PE).  Rounds can be placed very close together without compromising the armor's integrity.

Ceramic armor can be multiple-hit, but incoming projectiles damage the ceramic strike face and open up vulnerabilities in the armor... especially since ASTM standards for ballistic "testing" are just 2" apart. 

Steel can be multi-hit, but there are nagging "spall" (bullet fragmentation) and weight problems.

PE is light, it's available in soft, medium and hard resins (design considerations) and it can be layered to meet the threat. Full press manufacturing techniques at 3500 psi+ will lead to tighter consolidation versus, say 150 psi autoclaving with PE only armor (IMO). It can be manufactured in an array of shapes and sizes. Some quality PE is manufactured in the USA (Dyneema HB24, HB50). Others are manufactured in the USA but with high quality Chinese fiber (Honeywell Spectra 3780, Barrday UP 771). Certain Chinese 4-ply UD built with water-based polyurethane resins are also some of the strongest in the world.

The "problem" with RF2 PE "only" plates is their resistance to steel penetrator and fast moving FMJ ball rounds in the RF2 0101.07 protection category (M855, M193).  Our testing has indicated that you have to get most PE layups to around 30mm to reliably stop these projectiles.  If you can accept a thicker plate (1.18"), then 100% PE armor could be the right choice for you. 

As far as ceramic goes - my experience has always been that a high-quality aluminum oxide is much more adept at stopping multiple hits than other, more brittle ceramics.