My first was Lake Placid but it is a tough course and a crowded swim start and I would recommend it if you are confident in your race abilities.  Louisville was a great race because the swim has a staggered start and the rest of the course is rolling hills.  All Ironman races have good support and are run smoothly, so I would pick based on convenience/location and the difficulty of the course.  Good luck and congrats on deciding to move to the full distance!

Try Beach2Battleship in Wilmington, North Carolina.  I've done the Half there, but lots of my friends have done the Full IM there, and it's touted as a great first-timers race as well as one of Triathlete Magazine's Top Ten Ironman Distance races.  Flat bike and run courses.

Ironman Florida and Ironman Arizona are both considered "Beginner Friendly" Ironman events as well...but b/c of that they are hard to get into b/c so many folks want to be there. Good luck!

IMAZ was my first and only one so far.  I picked it because I stress out a bit before races and I knew traveling would be too stressful.  I wanted to be close to home so I didn't have to worry about shipping my bike or forgetting something.  Plus I was familiar with most of the course from previous races.  I say if you're doing an Ironman pick the course that is most appealing to you and will keep you motivated as you do your training.  I knew that by doing a race close to home I would have lots of friend come to cheer me on.  Having so many friends on the course cheering with signs and banners everywhere was incredible and kept me going when I was tired. For me that was one of the best parts of the race so I'm glad I stayed close to home.  Now that I have the first one under my belt (and most of my friends will probably not sign up for an entire day of cheering again so soon) I'd probably venture out and travel to one in a cool location.  Best of luck with whichever race you decide to do!