ARM is pretty orthogonal, though arm64 is less so. RISC-V is very orthogonal, but it sucks to program for it in assembly as the instruction set is missing even basic features.

Sorry, but Z80 being "regular and understandable"?

    ADD  A,B     ;8 bit add, B to A
    ADD  HL,BC   ;16 bit add, BC to HL
    ADD  B,A     ;invalid! destination can only be A or HL

    ADC  A,B     ;add with carry
    SBC  A,B     ;subtract with carry
    SUB  B       ;subtract (A is implicit, same for AND,OR,XOR)

    LD   A,(HL)  ;load A from memory at HL
    LD   B,(HL)  ;load B from HL
    LD   A,(DE)  ;load A from DE
    LD   B,(DE)  ;invalid! must be reg=A or mem=(HL)

    LD   A,(var)  ;load A from variable
    LD   HL,(var) ;load HL from variable
    LD   DE,(var) ;extra opcode byte, 4 cycles slower
    LD   B,(var)  ;invalid! must be A or register pair

    JP   (HL)     ;load PC with contents of HL register, NOT MEMORY!

    EX   DE,HL   ;exchange DE with HL (no other registers allowed)
    EX   HL,(SP) ;exchange HL with top of stack (no other reg,mem allowed)
    EX   AF,AF'  ;wtf were they thinking?
    EXX          ;should be EX BCDEHL,BCDEHL' for consistency :)

JP (HL) seems to be a disambiguator vs. JP HL (jump to label HL) and the register swap instructions, while quirky, seem justifiable as well.

I would just add SUB A, B (etc.) and call it a day.

edit: looks like the Z800 was an improved/orthogonalized (and pipelined!) extension of the Z80 that maintained software compatibility. And it looks like the SUB A, B format is supported by eZ80 assemblers, though A is still the only valid 8-bit accumulator.

I remember the Z80 in relation to the 8080. I might have misremembered but I thought Z80 allowed some combinations that 8080 could not do. It could also be that the assembler/syntax itself was nicer.