Best first Ironman race

I did Florida as my first just because it was the latest (at the time) in the season and gave me the most training outside. I can say it was a great race and the ocean is beautiful down there.

KY:  Freshwater (river) swim.  More hills on the bike (and I think the run) so you'll be in and out of aero.  Expect HOT and HUMID weather.  August means more daylight.  Relatively newer addition to the 140.6 series, so still building volunteer/community support (but good from what I hear).  Hasn't sold out quickly yet. 

FL:  Ocean swim.  Pancake flat bike/run (I guess there are a couple of overpasses to get across).  Because it's flat, expect to spend a lot of time on the aerobars and it can be windy.  Slightly earlier bike cutoff (I think because it gets dark earlier).  Well established race with good volunteer/community support.  Has sold out quickly in the past. 

Figuring the bike is the biggest part of the day, which do you prefer?  Hills and variety or wind/aero?  As for heat and humidity, if you can handle training in Georgia all summer, it shouldn't bother you too badly. 

What he ^^^^^ said.  It really depends on your  abilities.  If you have a choice personally I would choose which bike course suits your best.  The swim will be over before you know it and if you don't make it off the bike the run could be completely flat with the wind at your back and it won't mean a hill of beans.

I am planning on doing IMKY 2010 mainly because of the distance to my house.  Other than that I like the TT swim start (if it stays that way).

Of course you have to look at which course best suits your skills, that being said here are some other things to consider.

I did IMFL in 08 because it was close to home.  I had a fantastic time and don't hold any negative thoughts about my time there.  If asked I would do IMFL again because it was a great race and well set up.  BUT,there is absolutely nothing else to do in Panama City Beach Florida at that time other than the IM.  Nothing. 

There is a nice volunteer turnout but that is the case at most IM's, other than the volunteers there is a TON of alone time out there especially on the one loop bike course.  The bike course takes you through rural areas with very little fan support so your out there for a while with nothing but your thoughts and the freakishly large packs of drafters.  As you know the IMFL course is flat and fast depending on the wind and Louisville will be a tougher course but there are a ton of other things to do in that area.  Louisville's fan support cannot be any less than IMFL.

So, if you're looking for a place with options to pass the time or a large turnout of fan support i'd steer clear of FL.  But that's just me, I have been lucky enough to just do IM Japan where there was a ton of things to do and fans almost every step of the way.  So I admit that I am biased after doing such an awesome IM.

I've not done either race, but I'm leaning towards IMFLA in 2010 because of the heat in IMLou.  I work in an office and have a hard time gaining heat tolerance with mostly early morning training.  I've ridden on FLA 20 further west than the IMFLA course and it's lovely - flat and easy.