What is the most profitable type of investment?

[Ben Leary]

I have been reading up on the many different real estate investment niches. I was looking into investing in a 4-plex using an FHA but commercial real estate, larger apartment buildings, and mobile home parks all seem like they might be more profitable investments. I know it's a hard question to answer, but generally speaking, what is the most profitable investment I can make as a beginner?