kompleksa

The fence with komplika is worth building properly, because English lets the two words blur and Esperanto does not. Kompleksa is about composition: the thing is made of many parts that hang together. Komplika is about difficulty of untangling: the thing is a knot. A cathedral is kompleksa - thousands of pieces in a real structure - and it is not komplika at all, because you can see what everything is for. A badly written form is komplika and not kompleksa: two boxes and no idea which one to tick.

So the answer to "which one?" is: ask what you are complaining about. If you are describing how much is in there, kompleksa. If you are complaining that you cannot follow it, komplika. PIV keeps them apart on exactly this line, and Esperantists in practice do too, which is one of the small places where the language is sharper than English.

The noun is komplekso, a complex in the sense of a set of connected things - a building complex, and also the psychological sense that Freud's translators fixed in every European language at once. Kompleksa has no relation whatever to komplezi (to oblige someone), despite the letters.