National Aquatics Centre, Beijing

Last night I watched Usain Bolt do the most remarkable thing I have ever witnessed.

I, like many around me, could only laugh when I saw Bolt's new 100m world record flash up on the scoreboard. I was laughing at the words-fail-me joy of it all.

This morning, I have just seen Michael Phelps claim his eighth gold medal of these Games and 14th of his career.

So which performance was better?

Stupid question, right? Yes, of course it is. But it's not going to stop me from posing it, just as it won't stop sports fans all over the world from debating it.

Comparisons may well be odious but they're also addictive and inevitable. We simply can't help it and, like Pringles, once you've popped 'em you just can't stop.

While the "Baltimore Bullet" lacks "Lightning Bolt's" sense of theatre, Phelps' record-breaking display was undeniably magnificent. And it's not his fault he couldn't coast to the wall, while smiling for the cameras and beating his chest - he was swimming the third leg.

But for me, Bolt's performance was the pick of the two. It had everything. I won't go over ground my colleague Tom has already covered, but I have never seen anything like that before.

Of course, it's not just one performance with Phelps, is it? You are really looking at the body of his work, in every sense.

Gold number eight - a lucky number here, in case you missed that - came in the 4x100m medley relay. The US has won this race at every Olympics it has contested, so for many pundits this gold was a "gimme".

But that was always being a tad dismissive of the Australians and Japanese, and Phelps himself never talks about a gold until it is around his neck.

As it happened, the race was a classic, and the 23-year-old phenomenon's part in it was decisive.

Swimming the butterfly leg, Phelps took off from the blocks in third place after superb breaststroke shifts from Japan's Kosuke Kitajima and Australian Brenton Rickard.

No matter. Phelps simply steamrollered the field as he did in all five of his individual wins apart, ironically, from the 100m butterfly.

His split was three-quarters of a second faster than the next best swimmer and a US victory was now in the capable hands of Jason Lezak, the man who kept the Phelps' record bid on track with a stunning final leg in the 4x100m freestyle.

And that was that. Cue richly deserved celebrations, massive grins and the Stars and Stripes for the 12th time in the Water Cube this week.

Did this, though, elevate Phelps to a new level? In beating Mark Spitz's 36-year-old record of seven golds from a single Games, was he now more than just the greatest swimmer of his generation?

For Spitz he is. The American legend had already crowned his successor before number eight was in the bag.

"He is the single greatest Olympic athlete of all time now," said Spitz, speaking to US television on Saturday.

"He will probably be the single greatest athlete compared to anybody in any century - the 20th, the 21st, whatever."

That's a big claim, as "whatever" is a very long time.

Phelps is certainly the greatest swimmer of all time. And he would be justified in being miffed if he doesn't get the world athlete of the year accolade when those gongs are handed out.

But is he really what Maurice Greene has tattooed on his shoulder - the Greatest Of All Time? How can you possibly define that? How can you compare the apples and pears of so many different sports and eras?

You can't. But it's not going to stop people trying.

I'm not going to attempt to come up with definitive criteria for judging this or suggest a list of candidates - I'll only forget something or somebody. I'll leave that to you.

What I will do, however, is suggest where the biggest problems will come in really nailing this down.

The first major head-scratcher is the barriers to entry for each sport. To put things crudely, far more people can aspire to running fast than swimming fast.

Rightly or wrongly, swimming is still, by and large, a middle-class sport done well by a few wealthy countries. Yes, there are exceptions to this and 21 different countries that did win medals in the pool in Beijing.

But the fact remains over half of the 96 medals on offer were won by Australia and the US. Throw in China, the rest of the G7 and Russia, and you're up to more than three-quarters.

There will be a far greater spread of medals in athletics.

Another way of looking at this is the availability of opportunities. Even in a wealthy country like the UK, there is a postcode lottery element to swimming success due to our shortage of 50m pools and the concentration of top-notch coaching around those pools.

And perhaps another way of looking at this is to compare Phelps' achievements to other dominant sportsmen and women in fields that require a high degree of coaching expertise and technical input.

Putting aside the inevitable "are they really sportsmen, though?" claims, Tiger Woods is as dominant in his sport as Phelps is in his (and will break all of golf's records before he's done), and Michael Schumacher was one helluva driver.

But can "athletes" like that ever really claim to the "greatest"? What they are doing is such a specific thing and is only open to a minority of the global population.

It's a tricky one, however, because you cannot completely dismiss the achievements of somebody from one of these more esoteric disciplines just because not everybody has had a turn. Phelps, Schumacher, Woods and many more besides could all still be the greatest.

But it's a difficult argument to make stick for most people, which is why the majority inevitably come back to more accessible sports.

And why whenever I'm asked for my greatest sportsman of all time, I usually say Diego Maradona. Who, strangely enough, is rumoured to be in town and desperate to meet Bolt.

Fair play to Phelps, though. Perhaps we should postpone this debate until he's done. He's not finished winning Olympic medals yet, and neither is Bolt.

SO PEOPLE WHO'S PERFORMANCE IS THE GREATEST PLSE DNT BE BIAS!!!