.com is probably the best, simply because it's the most popular, the most universal...and as mentioned, people will remember that easier, or just default to it. If you can't have the .com that you want, and have to use a different extension, I'd suggest not using one where the .com version will lead to a competitor.
For example, if you can't have Indyphotographer.com because another photographer is using it. Don't just use Indyphotographer.net, because a portion of your advertising targets and clients will go to the other site, instead of yours. But if the .com version or your .net, is something irrelevant to you, that's not as bad.
Either way, something short and easy to remember is the trump card.
On a side note, it has come to light that any website operating under .com, .net, .org (and several others) are subject to intervention by the US government. Even if the company operates outside the US, the companies operating those domains is still inside, so they can shut you down. The gambling site Bodog had it's .com address seized, but it's Canadian (.ca) and other 'foreign' addresses were untouchable.
Not something we are likely to worry about...just a tidbit of info.