When you play on story mode and get new monsters available for purchase, here are the ones you will and will not want to buy.
The ones you will want to buy:
Mecha King Ghidorah
Who you shouldnt buy:
I know that some of those guys you may want to buy (especially Obsidius), but they are only good when you fight them because they spam. Or in some cases because they have better defenses. But, the ones that you should buy, are some of the best in the game.