First off, with a few exceptions there are no bad first ultras. A flat road course would more closely resemble the races most runners are most familiar with while a more arduous mountain race would provide more walking breaks and the scenery would lift your flagging spirit from time to time.
One could consider many factors in choosing a first ultra. If I were to suggest one as most useful it would be familiarity. This familiarity can be either preexisting or learned. If you are fortunate enough to live where that are mountain trails out your back door and there’s a race on those trails, run it! If, like me, you live within an hour or two of trails that are home to ultras, consider one of those races. You don’t need to run the race course every day to know it well. Rather, consider making the course the site of some of your long training runs.
If you don’t live near or can’t get to any ultra locations, consider talking to others who have run ultras. Find out as much as you can about a particular race. (In fact, this is a good idea even if you can run some or all of the course before race day.) Even if you’ve run for years, it’s great to know strategies for approaching the course on race day, to know where to be prepared for the heat or the cold, or to know how much water you should carry between particular aid stations. Before my first 100 miler (Western States 2004), I went to a briefing three of my clubmates put on. Before ever arriving in Squaw Valley, I felt like I was veteran of the race. I knew the course and how to approach it. Learn from those who have gone before you.
Now that you’ve chosen a first ultramarathon, read our guide on Ultramarathon Training.