I second IM Lou, again excepting the fact that easy and IM don't really go together...
You really just need to figure out your criteria. I like the idea of flat courses in a short race too, but a flat bike course means your legs never get a break, you spin the enitre time-- rolling hills means you get have breaks on every downhill-- which I found to be a nice mix.. At Lou, the first ten miles out and the (same) ten miles back are flat, so that was a nice warm up before the hills and nice coast back into t2. Also, the course isn't too windy, so I was able to put on some race wheels that helped (you won't want to do that at Coz--in fact, disc wheels arent even legal b/c the wind is so bad!)
I was deathly afraid of mass swim start, so I really wanted Lou because its the only IM with a time trial start. That was kind of neat. Also first 1/3 of swim is upstream, next 2/3 is downstream-- thats pretty sweet!
As for timing and heat...I would pick 90 degrees over 30 any day, so I loved the timing of the year for Lou. Also, it meant I finished the season relatively early, but wasn't ramping up when it was still winter, because my training plan really didnt start til early april.
As for the run, Louisville is two loops, pretty darn flat course. This is really a critical piece if looking for an "easy" IM-- because after a whole day out there, you will feel even the slightest incline on that run, so if you pick a race with a non-flat run, it will hurt!
So as you can see, we all have very different ideas about what defines easy, but what you need to think about is:
1. How well do you handle heat? Cold? Think about in context of both when the race is and what time of year you would be training
2. Hills versus flat on the bike
3. wind on the bike
4. Swimming ability and comfort-- time trial (louisville is the only one) vs. mass start; also, current (upstream/downstream); water temperature (wetsuit legal vs. not?); ocean vs fresh water (difference in buoyancy).
I can comment on IM Cozumel, AZ and Louisville.
Cozumel - I liked the fact that you can see in the clear water and that is something huge mentally for me. The mass start didn't bother me as much b/c you could see what you were about to run (or not run) into. I didn't have much of a current the year I did pushing me back in but I know the first year they had it there was a nice current. There are also little sea lice that prick you a bit but knowing that ahead of time helps so you aren't wondering what the heck is going on.
It may be a flat course but be prepared for some wind on the bike. I love flat runs but it was hot and I couldn't cool off so didn't enjoy the run on this course.
AZ - Be prepared for a colder swim. I used neoprene cap and booties so that helped a lot. The water is pretty murky. The bike has a false flat and the year I did it I remember how awesome it was having the wind push me back in the 1st and 2nd loop. The wind didn't push me is as much the 3rd loop and had changed directions a bit. I loved the run! The three loop figure eight course was amazing for my dad and Ron to find me multiple times on the course!
Louisville - Probably my favorite IM course but I lucked out with the weather. It wasn't crazy hot the year I did it. The TT swim was definitely nice except at times when you'd run into someone since you wouldn't always be swimming with people around the same pace. You just have to make sure you get in line early enough to get a decent place in line. I was luck to find Hillary in line and she let me hang with her. :)
Great rolling hills on the course and nothing crazy steep. Riding out on River and to Poolesville and beyond was great prep for this course. Run was nice and flat! Awesome finish line and there's a great sports bar right next to it that had big flat screens with the finish streaming live. I flew to this race but I know some people drove so it's doable if you don't want to deal with shippign your bike etc.
Check out Beach to Battleship - late Fall in NC. I've heard only great things about this race.