This is really a ridiculous question. Why would Singapore want to invade Malaysia? It's like a small snake trying to gobble a cow...

Therefore, the question is really whether if Malaysia invades Singapore, would they win? Assuming no outside interference, most likely Malaysia would prevail. However, that assumes that this would be politically and economically feasible. And it's not.

After the Iraq invasion of Kuwait, I doubt anyone would be that dumb. Besides, Singapore has no natural resources to speak off and most of the wealth is invested overseas. What's the point of an invasion?