Nielsen/NetRatings, released recent market share data:
Google: 47.7%
Yahoo: 21.8%
MSN: 11.3%
AOL: 7.2%
Ask Jeeves: 6%
All the other SE's combined have approximately a 1.5% market share.
But, don't take that into consideration alone. Place tracking code on your URL's and webpages, and track. Test, Analyze the data. You might find that enhance interactive is the best converting SE, and not Google. You might get the best ROI on MIVA. Who really knows? It's different in each industry. BigResearch just conducted a study and found that in the Apparel industry the best converting SE's are in this order:
1. Yahoo!
2. AOL
3. MSN
4. Google
5. Ask Jeeves
So, if you are just interested in traffic volume, then Google is the answer. But, if you want to know which converts the best, then test and analyze for your individual results.
Hope this helps:-)