Right now the leader is still Wizards of the Coast.
Fans worldwide wait and anticipate WotC product more than any other company. They have the best magazines, the best presentation, and plenty settings to choose from.
It's hard for me to say in any given order who follows but I think AEG and Kenzer move back and forth between number 2 and 3. AEG's strong point is their diversity in product. Spycraft, Rokugan and soon Swashbuckling Adventures and Dark Sun. Those cover a large range of material with a professional presentation. Kenzer's Kalamar is one of the best D&D settings out there with a consistant release schedule. I could have easily choosen this setting to game in.
Number 4 for me is Fantasy Flight. I love Dragonstar and the Legends & Lairs line of books. The one thing keeping them from challenging the number 2 or 3 spot is lateness in releases.
Number 5 is the Sword & Sorcery line from White Wolf. Scarred Lands is cool and all but it doesn't translate very well into other fantasy gaming settings. Necromancer Games' "first edition feel" means I don't want to play it. That's just me though so don't take it the wrong way.

Malhavoc stuff is awesome and recommended but isn't released in any great abundance. I love the stuff from Fiery Dragon but they're leaving Sword & Sorcery. :/ When Everquest comes out Sword & Sorcery could possibly shoot up a couple notches to 2 or 3. We'll see.
Number 6 is Mongoose. What hurts them the most is they just put out too much stuff. Four to five releases a month is too much. Slayer's Guide to Girl Gamers? Come on guys.
Number 7 is Green Ronin. Great stuff over all and consistant. If there is a challenger for Mongoose it's Green Ronin. As Green Ronin is releasing more quality stuff I think they can start moving up the list.
It's pretty much a free for all after that. There are some outstanding companies out there that are doing some great things. Mystic Eye and Thunderhead both are carving nitches for themselves. Troll Lord is cashing in on the Gygax legacy.
The trick is to do unique things or do things very well. Hopefully D20 Modern will allow many companies to do some very cool things. Hang in there publishers! It's getting nasty but there's still hope.
~Derek