I'd have to say the Speedway because it takes up SOOO much space and it's not Tomorrowland-ish. It's too carnival-like and just not up to Disney's standard. All that room could be used so much more affectively. They could put a great attraction there to rival Space Mountain!
I totally agree with this! The only potential challenge with regard to putting in a replacement attraction in this space is I wouldn't want a huge show building to ruin or disrupt the TL skyline. But I absolutely agree it is a huge amount of wasted space for an attraction that is just no longer relevant.
Some may want to flame me for this, but I also would get rid of IASM. They could replace it with a really cool dark ride, or even an e-ticket considering the size of the show building the ride is in.