Best is a tough word to define. The two major distributions (SuSE/Novell and RedHat) are acceptable quality, although...
SuSE has somewhat of a better reputation in the mainframe arena due to a bit of a head start in working with the platform. Red Hat delayed a while before committing, and thus is somewhat behind. RH has also had some problems with field personnel criticizing the mainframe platform and attempting to steer customers away from it. Recent releases of SuSE have had some QA problems in the installer that have tripped a lot of people up. Debian (probably the largest of the 2nd tier distributions) is also well-supported on the mainframe, and has good acceptance in the community as a viable alternative.
If you define "best" as "largest number of commercial applications supported", then SuSE is arguably well ahead of Red Hat and Debian. SuSE/Novell has made the most of their initial market lead, and seems to be consolidating as the dominant mainframe Linux distribution. On the other hand, Red Hat's successful market strategy in the Intel space has lead to a number of enterprise Linux contracts that have many large institutions deploying Red Hat because it's included in their enterprise agreement with Red Hat, and they want consistency across multiple platforms.
Both SuSE and Red Hat charge a premium price for their distributions, and licensing can be somewhat tricky to understand and manage. Debian is freely-available, but is not certified by many of the ISVs for commercial application use (although in most cases, the applications work just fine; the ISVs just haven't tested on Debian, and are thus unwilling to make guarantees).
So, best is a nebulous word. If you want to get your feet wet with Linux on the mainframe, Debian is a low-risk, low-cost way to do it. If you plan a commercial application deployment, check with your Intel folks and see what they use. There's a small advantage in being able to use common experience, and it's a religious battle you don't have to fight if they already prefer one distribution over another.
Do IBM apps (such as DB2) run better on SuSE or Red Hat or something else?
IBM and the ISVs have more experience with SuSE, but the applications essentially run the same and require the same tuning on all three distributions. Most of the tuning parameters and application performance are the same whatever distribution you run.
Which company has the best support?
Keep in mind that you do NOT have to buy support from the distributor, which is a good thing, as both Red Hat and SuSE are fairly light on mainframe talent. You'll probably find that the 3rd party providers and the linux-390 mailing list are much more capable of useful support than the distributors are. Some of my clients report mixed results with the IBM Linux support offering, although it seems to depend greatly on what part of the world you reside in. In any case, consider specialty providers as well as the big players; for the mainframe Linux world, they are much more capable and have much more detailed knowledge of the mainframe Linux and VM marketplace than the distributors do. This is important, because many of the assumptions made in the mainstream Linux community are flat-out wrong in the mainframe world. The special knowledge is worth the price in terms of success and in terms of reliability.
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