You'll probably find lots of threads in the archives arguing this, but here's my personal take on it as of now in my order of preference.
.com - Far and above the most preferred, but good names are hard to find and bad ones hard to sell.
.net and .info - I currently think they are about even and each fits certain uses better. Two very versatile extensions based on intended use.
.org - good, but it still has the "non-profit" or personal atmosphere which is good in some instances and not so in others.
.us or your own ccTLD - .us is also an up and comer rising in popularity. in that it the extension only means you are in or deal with the US market, it is extremely versatile and can be used for anything from business, information, personal, etc.
.biz - an up and comer with some good one and two word usable domains not too difficult to find or afford. Names must fit the extension though.
.tv - recommissioned ccTLD of the island country of Tuvalu - useful if you deal with television and can't get the .com. Expensive! Otherwise, why?
.ws. .bz, .cc, and soon .pw - more recommissioned ccTLD's - okay for personal sites, and maybe okay for very strong single keywords. buy to develop, but not to resell (at least not in the short term).