As for a "good" first Ironman, there are several variables involved. And hopefully you realize that there are also no "easy" Ironman events. There ARE some specific considerations and factors that WILL influence which one you pick -- and they're applicable whether it's your first or my 19th coming up..
One consideration is WHEN the IM is in the year. The longer out it is, the more time you have to properly train. The closer it is to when you select it, the less preparation will be available; the unwanted solution to not having sufficient time to train for the IM is that most IM events necessitate signing up the day after the race for the following year! NOW you have enough time to train.....
WHERE the IM is located will also be a consideration; travel expense restrictions can dictate which IM you might choose. Doing one close ot home enables friends and relatives to come and cheer you on; one overseas can be chance to travel to new places and vacation afterwards. A foreign or unfamiliar venue can be a plus -- look at it as an opportunity to experience a challenging event IN an new/exciting locale.
The COURSE should be a consideration -- does the one your interested in take advantage of your strengths (i.e., it's a hilly bike and you're strong on the hills) or will it seriously "test" your weakness(es) like a historically rough ocean swim or hot run (both of which you may not be comfortable with)? The downside to a course can be prepared for and negate YOUR “LIMITERS” by the "specificity" of your training -- doing ocean swims if that's what the course is, biking hills similar to the race course, etc. Knowing the topography of the IM course can be simulated on the CompuTrainer, treadmill, or with open water swims. Research what the topography will be on the course you sign up for and train under those conditions.
Don't forget nutrition/hydration considerations, doing races that lead up to the IM, injury/overtraining avoidance, using a HRM or Power Meter to vary workout intensities. having a personalized training plan, and supplemental activities like strength training, stretching/flexibility, specific workouts and/or periodic testing of gauge progress, etc. .Training properly for ANY IM entails more than just swimming, cycling, and running. Other factors affecting which IM you choose are cost, location, and time of year. Geez, nobody said it’d be an easy choice or “journey”….
And remember that an IM is more of an endurance event than one based on speed. You have cutoffs that you have to meet on each leg, but the underlying goal is twofold: 1) First you finish then 2) you look at the clock......
A "good first-timer Ironman" is one that 1) you approach with respect for the distances with 2) training laid out in a realistic, adjustable training plan which 3) will help you develop the pace and patience to "go the distance". An IM is a test of mental AND physical strengths -- when you cross the IM finish line, you won't be the same person that started the race......