The IF form would be fastest I think.
INLIST() suffers from the limitation on the number of parameters you can pass to a VFP function, so it's not infinitely exendable.

Watch out for SET EXACT, it affects how a = "a" is evaluated.
If you really, definitely, want to know if a = "a" then use a == "a", otherwise you can get caught out.

Try this in the command window
? SET("EXACT") && so you can set it back afterwards :)
SET EXACT OFF
a="ab"
? a="a"
? "a"=a
? inlist(a,"a")
? a=="a"
SET EXACT ON
a=""
? a="a"
? "a"=a
? inlist(a,"a")
? a=="a"

Which will sometimes get you in a heap of trouble.