What letters of the alphabet are followed the most by the letters ei?
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.