There is none.
There are two real ways that people tend to interpret math questions as 'complicated'.
1. It's really long and a person's mind immediately goes "HELL NAW I'M NOT DOING THIS'
2. They don't know how to do it. People tend to assume something is 'complicated' if they do not understand it - for example, if I gave you a a basic question around complex numbers (For the record this does not mean simply "complicated numbers". It refers to values like I which is an imaginary number denoting the value of √(-1)), people who don't understand would probably go "WTF IS DIS?" while people do will just be like "Lol dis is 2ez4me"
How complicated a question can be is easily determined from person to person.
A person who is good at maths can easily change what some may consider a 'complicated' question into a simple one by use of a theorem. Likewise, some people tend to associate the word 'complicated' with extremely long questions, that might just be basic addition or multiplication, while other are able to react immedietly and be able to see what kind of question it is, what knowledge it requires, and from that make their own judgement on the difficulty.
Lets take the damage calculation formula
"Dang mate what the hell is that?"
But if you just look at it, you can immedietly see that all it requires is knowledge of Multiplication and Addition (and BODMAS, some of you may know as PEMDAS or something. Varies country to country).
Now lets look at the Experience Gain Formula
"WHAT THE HELL DO ALL THOSE LETTERS MEAN?"
"MY HARDEST SCHOOL QUESTIONS GIVE ME 3 PRONUMERALS TOPS AND YOU GIVE ME LIKE 6???"
But if you just take a quick look at Bulbapedia or something, and check what they mean, it becomes are pretty simple (possibly time consuming) equation. Sub in the values, you're good to go.
Anyway, Pokemon formulas consist of 4 things really - Multiplication, Division, Addition and Subtraction. Some of them look long and annoying, but in reality are about as difficult as that of a 4th grader math question.
There is no most complicated one. It may vary person to person, but they all utilise the same concepts.