Solved by Mike Schiraldi

solution: nuggest

Here's another way of looking at it:

 1  2  3
 4  5  6
 7  8  9
10 11 12
13 14 15
16 17 18
19 20 21
22 23 24
25 26 27
28 29 30
31 32 33
34 35 36
37 38 39
40 41 42
43 44 45
(and so on, to infinity)

You can get any number in the right column except 3 by adding 6s and 9s. So cross out the entire right column except 3. You can also add 20 to any crossed-out number and cross that number out. So cross out everything in column two below and including 26. Finally, by the same logic you can add 20 to the crossed-out numbers in column 2 and thereby cross out everything in column one below and including 46.

The largest number that's left over is 43. Incidentally, cross out 20 and 40 and your map is complete.