The one you enjoy the most and feel passionate about. 

Gary Vaynerchuk echoes this thought process when he talks about how real estate agents can differentiate themselves (a highly recommended video). 

Referring to your thoughts on GAMBLING, I think it is much more of a gamble to not pursue something you are passionate about so that you can have/keep "safe Job" for 30-40 years in hopes that you will one day have enough money to enjoy life.

All qualify for 1031 treatment so you don't have the tax bite and all can provide a source of residual income.The question is where does your passion lie.  

I am an insurance agent with a huge passion for all things canine.  

If this really is your passion let me know and I can get you an interview with a really great company in your neck of the woods, lol. 

Networking with other individuals who share your passion will open many doors for you.

Buyers please drive the area yourself, or have your agent video tape the areas while driving around the streets you are interested in.

It's always good to have a prior move in sheet, pictures or video to show the condition of the property prior to their occupancy. 

I saw a youtube video a while ago where someone was able to just draw an outline on a satelite map and he was able to get a mailing list for all the properties within his outline, of course now i cant find it