I use apple or mesquite depending what I'm doing. If it came down to one, I would keep apple on hand. DW got me an electric smoker last Christmas and we love it, it rides in the truck forward of the hitch just nice, and friends we camp with get excited when they see me unload it.
One of the best things I got for it was a good wireless thermometer. It has 2 probes, one for the temp of the smoker and one to go in the meat. It has a satellite device you can keep close to monitor you temps, set alarms, etc. I found that the built in thermometer on the door was about 35 degrees off...explains a lot of issues I was having when I first started using it.
Edit: Link to thermometer