Board Thread:Questions and Answers/@comment-24731329-20151214233123/@comment-26574811-20151214235013

1-1 is cheapest, but not necessarily fastest.

1-4 is a good choice, with fairly low cost and very high boss encounter rate.

The first area with absolute boss encounter is, I believe, 5-2, which is not quite cheaper than 5-4. But if you are leveling ships, 5-2/5-4 could also be a good choice.