acf573 is right, It varies by lender and by state. Sometimes it varies by state within lender! For instance Citi will pull Equifax for part of the country and Experian for other parts. Very little rhyme or reason, IMHO.
My experience (in New York state) was that up until June, CITI was pulling TransUnion - which was great as they were the only inquiries I would have on TU.
But I was denied for the first time on my last application in June for too many inquires - reason was that CITI has CHANGED from using TU to Experian - which for me, is the same credit agency used by Chase, Amex and BofA.
Important info in deciding which cards to churn, when.
It varies so much only you can answer it yourself, unfortunately.
What someone else sees is not what you'll see.
Some peole are seeing Citi from Equifax before and TransUnion now.
Other people are seeing Citi from Transunion before and Equifax now.
For whatever reason (be it what state you're in, what locality your'e in, your SSN divided by 3, or the phase of the moon) the results vary from individual to individual, and even for a given individual from one card to another!
By "answer yourself", I mean get your credit reports. If you've not done so before, you can get one of each (of the three) free at www.annualcreditreport.com. (If you get all three at once, though, you'll have to wait a whole year before you can get another of any of them. If you don't need all three at once, you can stagger them, as long as each one of the three is only once a year.)
If you're concerned about this on an ongoing basis, you may unfortunately need to pay to get credit reports more frequently than once a year per service.
I used to work for a couple of different issuers. For one, we pulled according to geography, in a single acquisitions campaign we would have customers with records from each of the 3 bureaus. For the other issuer, we were much smaller and only used TransUnion but then switched to Equifax when we had an issue. When I left, we were negotiating with each of the 3 to see who would give us the best price if we gave them all our business.
I applied for a Chase business card and they pulled my personal credit from Experian. I was declined due to "too many recent requests for credit". I guess I finally over did it! I pulled my report for free and counted 12 inquires in 2008. I already have 5 Chase cards along with checking, savings and a home equity line.