This article will be about a few of the major magic sites out there. I'll cover both past and present but will likely completely forget a few important ones because I either had never heard of them or because they just slipped my mind.

The Dojo:

I may as well start with the original "best" Magic site. This site basically invented the internet Magic community. This brought both good and bad things to the game of Magic. On the positive side it gave tournament players access to as much Magic information as they could possibly want and gave more casual players a look at competitive Magic. On the negative side this also created the netdeck which many people consider the death of originality in magic. It only takes one look at Tsuyoshi Fujita to realize originality is far from dead but playing a Tooth and Nail deck 5 cards away from a version that just won the latest Pro Tour in a casual environment is definitely not a nice thing to do. The Dojo closed down years ago but you can find some archives here and here (reading articles about how the new 6th Edition rules will kill Magic is some quality entertainment btw).

magicthegathering.com:

The only official Magic site and also the one with the most traffic (StarCity is second). Easily the best place for casual magic (especially the boards) and now has pretty good competitive articles as well (mostly due to Flores and Zvi). This also provides something you can't get anywhere else and that is that since it is an official site half the writers are actual Wizards employees. This means you get a closer look at what goes on inside the company and also most news is released here first (unless it gets leaked early in which case mtgsalvation usually prints it first).

The Sideboard:

This was the official Wizards tournament Magic site. It had an occasional drafting or constructed article but was really known for being the official place for tournament coverage. It has since merged with magicthegathering.com as Wizards decided there was no reason to keep the sites separate. If you dig around in the archives in the tournament center most of the older coverage is from the sideboard.

MTG News:

Just a year or two ago this was the number one place for Magic related news. They were always the first place to have full spoilers and had very active message boards. It also had the first decent spoiler generator that let you search with many different filters. Eventually the boards declined to the point where every news post no matter how minor fueled dozens of people to declare the news to be "the death of Magic." This was understandable behavior in large things like the 6th Edition rules changes or new card face in 8th but they would have that reaction to literally everything. Removing Winter Orb and Stasis from the base set? Magic is now dead. Deciding Banding is too confusing to keep in print? Magic is dead again. Eventually the site stopped being the first to report news and eventually just started reporting things said on other sites a day or two later. It became the number one an almost bearable place for old Magic news. For a clue to how dead it is the last update was on Aug 3 and it was updated a total of 3 times in July. Even the spoiler generator is broken now.

MTG Salvation:

This site has pretty much taken over for the now dead mtgnews.com. It is now generally the first place to have full spoilers and has pretty active message boards. I really don't visit it for anything beyond early spoilers though.

Brainburst:

Once upon a time a bit after the Dojo closed this became the best place for competitive magic information. This is where I first started reading about tournament magic. The articles were very high quality and informative. They had all the big names such as Zvi and Kai. They eventually started a premium service which helped them afford even more quality writers. Eventually though the quality started slipping. The articles kept getting worse and worse and many of their big name writers started leaving. The site is now practically a joke. They have like one quality article a month on the free side. I'm not sure how the premium side is but I've heard it isn't much better. To give you a clue of how bad it is earlier this week there was an article about building a R/W beatdown deck. The deck was one of the worst decks I've ever seen on a tournament magic site. It had three copies of Goblin Piker in the deck but had Hearth Kami in the sideboard. It also had 2 copies of Warrior's Honor in the deck but no Roar of the Kha, maindecked Reverse Damage and Path of Anger's Flame, and running multiple copies of Flamewave in a deck with no mana fixing. Also it was a post 9th Standard decklist with Boil in the sideboard and thus isn't even legal. That said I usually at least take a glimpse at the site each day as there are occasionally good articles. They are just quite rare.

StarCityGames:

This is the current leading site for competitive magic and probably second for casual magic (after the official site). The articles are often very high quality. As soon as I saw that Jamie Wakefield was a premium writer I signed up for three months of the premium mostly just to see if it was worth it. Turns out it was and when my subscription ran out I signed up for another year. There are generally at least 2-3 amazing articles a week and even the others are often entertaining and/or informative. Even the free side has plenty of quality articles. Also this site in number one in terms of just the quantity of articles printed. Most magic sites print 2 articles a day. StarCity prints around 4-6 counting both premium and non-premium. Also since the premium service popped up the prices of their online store have been getting much better. While everything used to be hugely overpriced a lot of it is much more competitively priced. There are many cards that are even comparable with cardshark now.

The Mana Drain:

This is a Vintage focused message board community. I rarely if ever read anything on here and use StarCity as my source of Vintage information but most of the major Vintage names hang out on these boards and if you wanted to give Vintage a serious try this would be a good place to start. There are also more casual sections of the boards that generally contain the crappy Vintage decks but if you want casual boards magicthegathering.com is a much better place.

The Source:

This is supposedly the number one place for Type 1.5/Legacy information. Not that that means a whole lot at the current moment but with Gencon coming up (which contains the Legacy world championship) in a week or two as well as the fact that Wizards is officially supporting the format next season (at least on a trial basis) means this will become more important later.

There are certainly plenty of other magic sites out there but these are the ones that provide(d) the best information. I read everything magicthegathering.com and StarCity and skim articles on Brainburst daily. I'll look at new spoilers on MTG Salvation but generally don't pay attention to articles or the rumor mills.