Beyoncé is one of the biggest artists of the century so far and the Single Ladies video is one of her most recognisable images; it has been parodied in many cultures all over the world by people young and old. This in itself gives the video the potential of becoming iconic, and obviously I realise time is an important aspect to consider and that we can't truly know if it is iconic for sure for another 15-20 years. I truly consider it one of the most recognisable images in pop music this century and considering it hasn't been in the charts for years, yet continues to be parodied years and years later, I think it has a very good potential for becoming an iconic image for music in that time period.
Do I personally think in the longterm Single Ladies will be remembered more than Heal The World? Yes I do. It was the bigger single of the two and has a far more recognisable image to it's name. But yes, like you said, only time will tell. We will have to wait to see if it could fulfil it's potential and becomes an image recognised even by future generations.
I would not however compare the song to Billie Jean, Thriller or Beat It (which, as you might've noticed, I didn't). Singles Ladies was a hit, but those three? They were megahits and I don't think even Beyoncé herself would compare Single Ladies to them (especially knowing how much she respects and recognises Michael's influence). They're the three most famous songs of one of the most famous musicians in world history, so... yeah.