What's best: E.max crown, Empress, or even PFM crown for severely discolored anterior tooth?

I would recommend that you consult with a dentist experienced in cosmetic treatment.  My favorite material is Emax and used 90 percent of the time on front teeth.  If the tooth is extremely dark and can't be blocked out with porcelain I will go with a material called Zirconium.  This is a tooth colored metal that is both strong and aesthetic.  Best of luck. 

Alone none of these options will work well and will not look natural.   I have treated a situation like yours many
hundreds of times over my career and there are two possible solutions that will
give a very good result.  The BEST for a
tooth that has had root canal therapy is to use internal bleaching to bring the
tooth up to the same color as the teeth adjacent to it and then bond the
internal area of the tooth with a light colored composite filling material.  In many cases this is all that has to be done if
the basic tooth structure is adequate. If the tooth bleaches well but still needs a crown, then E-max or Empress could work well. 
The SECOND solution is to crown the tooth with a layered technique.  The inside of the crown immediately over the
prepared tooth consists of a thin layer of very opaque zirconia which can be
colored to match the final color desired. 
Over that is fused a layer of Porcelain which will allow the tooth to
have a very natural appearance and match the adjacent teeth exactly. I would
suggest you find a dentist that recommends the treatment options I have mentioned
without you directly asking for them.

Of course it depends on how discolored the tooth is.  With the right preparation, an emax crown can be very effective in masking discoloration.  The other option would be an anterior zirconia crown.  These can be shaded well and have a natural translucency.  They are very effective in areas where the only option was porcelain fused to Gold. 
As always, make sure you get a consult with a dentist well versed in these procedures.
Good Luck