Which cascade is safest? Which cascade in VeraCrypt is the safest?
(assuming same password, keyfile strength etc)
a] Serpent(Twofish(AES))
b] AES(Twofish(Serpent)

What could the difference be? I can't think of any and I don't uderstand why there are 2 options. Which one would you choose and why?
Both options for the 3-cipher cascade are equally safe.  

All three algorithms are strong with no known issues.  Veracrypt uses them in well-studied modes that also have no known issues.  

If you were worried about one of the algorithm implementations being vulnerable to some sort of side channel attack, then I would use that one as the last to be applied.  But in the context of most Veracrypt usage, someone who can mount a side channel attack, will probably just install a keystroke logger and steal the password directly.   

So bottom line:  There is no real difference, and you can arbitrarily choose one with no real impact.  

You also ask for an opinion about what the community would choose, so I'll offer mine:  The real threat is bad passwords or implementation bugs not weak algorithms, so I'd go with the highest performance single-pass algorithm.  A three cascaded cipher implementation is overkill.