A = 51.0;
B = 52.0
C = 10.0
D = 12.0
E = 1024.0

new Float:distances[] = { 51.0, 52.0, 10.0, 12.0, 1024.0 }; // Set an array with distances

new highest;
for(new i; i < sizeof(distances); i++) // Loop thru the distances
{
    if(distances[highest] < distances[i]) // Check if the distance is higher than the previous highest distance
    {
        highest = i; // If it is, set this slot to the currently highest distance and continue the loop
    }
}

// Once the loop has ended, 'highest' variable contains the slot that has the highest distance

printf("highest distance is %d (%f)", highest, distances[highest]);