Buy what you know how to make money with.

I like timberland investments, let me know if you are interested in help with this.

Checkout self-storage and I think you may be very pleasantly surprised.

I like multi-families, especially 2-unit properties because financing terms are so generous.  

Obviously, the financing terms are less generous on 3-4 units, but still very friendly when looking at the potential ROI.  Fewer units = fewer people to manage/lower potential for issues with tenants, too, so that's a plus.

Multiplexes. If this is your first depending on ROI I suggest a 4 to 6 unit complex to start.

I'd say multi-family hands down, much easier to manage and all in one location.

Much depends upon your local market. Multi-family buildings are more efficient to operate and manage.

Unless you are in a small market, I'd say neither.  The more specialized you become in a niche (or two), the more successful you will be.  You can become "the expert" in that niche and the go-to-guy.  I am looking for a condo specialist in another city right now.  I don't want a Jack or Jane of all trades.   That being said, if you feel the need the choose one, I would say multi-family dwellings.  Investors are more likely to become repeat customers sooner... or have other properties for you to sell.  

do some of both....