Well ok. The way I look at it, this answer is Google. As of recently, Google has started to become huge, starting things such as GooglePages, GMail, buying YouTube and merging Google Video, Picasa, Blogger, and plus they are honestly one of the best search engines as well, and now, they own their own cell phone which they launched to T-Mobile, the Google Android.
Microsoft has been big from the start, owning software on the majority of PCs out there (Windows). They've grown a little past the software industry by doing things such as making the Zune or starting services such as Skydrive, Club Live and Hotmail, and a while back ago when they started MSN (since the start of the internet) and MSNBC, but basically, that's just about it. MSN search did move to Windows Live Search, but even then, it's still the same search engine as it's always been, but it's still trying to rival Google in terms of the quality of the searches.
Yahoo I haven't seen change at all within the last few years. Sure, there's tons of services that Yahoo runs such as web hosting (Microsoft and Google do this as well) ShoutCAST, Yahoo Games, Radio, Weather, Sports, etc. However the search engine works very close to where Google's is. I've used some of the webmaster tools in Site Explorer on their site and I'd have to say they're very useful.
Really, since you didn't necessarily say if this was a search engine battle, I'd have to break this up somehow. If you're talking about overall growth within this year, it's going to obviously be Google. This is all thanks to GMail, buying YouTube and Google Android.Overall growth from day one of the company, would obviously be Microsoft. Growth in services since day one would be Yahoo of course.
So it's really hard for me to decide who is number one. But hey, it's what competition does to you.