Technically it could be 25, but 1 is the most complete answer.
1 is more consistent and symmetrical.
25 looks like it fits because the number direct opposite the smaller numbers are = to themselves x 5.
2 is opposite to 10 (with 5 in the middle as the multiplier).
But it leaves an obscure series of numbers without any consistent pattern or logic.
1 is a better answer because it evenly divides the top half of the circle into 1 to 4
and the bottom half into 5 to 20.
The top half cleanly multiplies by the middle 5 giving the answers provided below it at a clean 180degree break. And patterns the top half to integer (step+1) numbers below 5.
With the bottom half being multiples of 5.
Top half: 2,3,4,1 - shows a sweeping step increase from right (1) to outside and back in to the left side --> (2,3,4)
Bottom half: 5,20,15,10 - shows a sweeping step (+5) increase left to outside and back in right side.
e.g. <-5, <-10, <-15, <-20.