well, I'd hardly call it the biggest game in development. I'm sure more folks are looking forward to Doom3 and Counter-Strike condition Zero and Deus Ex2 and Duke Nukem Forever (which has been in development forever)
All of those combined
might be able to give HL2 a run for its money. DooM3 is an id game, they sell quite well, but not in the same class as what HL did(although the engine is sure to be a huge hit). The tech boards are filled with talk of DooM3 because of its technology, I don't see a lot of hype building around its gameplay(which is supposed to be very un DooM like).
CS0 looks like it could be good, but CS's main strength is the MP and online games still pale in comparison to single player games when it comes to market penetration. Also, with the way this game has been handed around, I know doubts are starting to creep up. CS0 should hit close to the same time as HL2, watch the sales charts between them.
DeusEX was a relative failure in terms of sales(compared to the games we are discussing anyway) no matter what level of critical acclaim was heaped upon it. They built a strong following amongst the hardcore gamers who were running 3dfx hardware at the time(the game was pretty much unplayable on anything else, and by the time it was patched decently the hype had long since faded), and will likely expand their fanbase with DEX2, but it isn't in the same realm as HL2.
DNF is considered dead by an awful lot of gamers. At this point a lot of people are thinking 'Daiktana' level quality. It has been so long between franchise releases, and this game has been in development so long, at this point only the very die hard are showing much interest in the title.
Half-Life 2 is pretty much without a doubt the most anticipated PC title right now(or, it will be when the mags hit the shelves and John Q Gamer hears about it). The only franchise that could topple that would be 'TheSims2'