Beyond "i before e, except after c!"

Solution 1:

The answer is r, largely due to productive prefixes like re- and pre-.

Per the OED, words with a letter followed by ei occur with this frequency for each such letter:

 981 r
 586 l
 518 h
 478 w
 394 s
 366 c
 349 v
 341 n
 315 t
 224 d
 149 m
 145 f
 140 b
 139 e
 124 p
 114 g
  81 o
  73 k
  67 u
  51 a
  50 y
  24 i
  19 z
   4 j
   2 x

This includes words like reification, reillumination, reimpregnation as well ones like Reichstag, reindeer, reine. Because hyphens don’t count, words like pre-install are included, since these can be spelled without the hyphen as well.

The main j-word is jeistiecor. There are also cross references (older spellings) of jeig for jig, jeine for jean, and jeit for jet.

The two x-words are exeime for exeme and hexeity for haecceity.