1
3
5 3 4
 6
5
5 3 4 4 5
3
1 1 1
5
4 4 3 4 4
5
5 5 4 3 2
3
1 1 2
5
5 4 3 5 5
 1
3
10 1 5

1
 1
-1
2
2
3
1
 1

import sys
input = sys.stdin.readline

'''

'''

def solve(n, fish):
    mx = max(fish)
    nm = n-1
    for i, f in enumerate(fish):
        if f == mx:
            if i > 0 and fish[i-1] < mx:
                return i+1
            if i < nm and fish[i+1] < mx:
                return i+1
    return -1
"""
def solve(n, fish):
    mx = fish[0]
    mx_i = 0

    for i, f in enumerate(fish):
        if f > mx:
            mx = f
            mx_i = i
    
    size = fish[mx_i]
    l, r = mx_i-1, mx_i+1
    eat = True
    while eat:
        eat = False
        if l < 0 and r == n:
            break
        
        while l >= 0 and fish[l] < size:
            l -= 1
            size += 1
            eat = True
        
        while r < n and fish[r] < size:
            r += 1
            size += 1
            eat = True

        if not eat:
            print(l, r, size, mx, mx_i)
            return -1

    return mx_i+1
"""

t = int(input())
for _ in range(t):
    n = int(input())
    fish = list(map(int, input().split()))
    print(solve(n, fish))