RedHat Enterprise Linux or CentOS, is a good starting point. But... the more you run it, the more you'll end up not liking it. Believe it or not EoL comes really fast for a Peg In ever moving wall kind of distro. RHEL if you want official support. Spendy though.

Unbreakable is Oracle's schtick. Also Spendy... but they probably have a migration assistant from Solaris. If you are moving away from Solaris because of Oracle and its shenanigans, then stay away from it.

SuSE Enterprise is very nice... it is typically is more up to date and doesn't have huge amounts of legacy and community problems the RedHat/Fedpra does due to the huge community. Support is spendy also.

Ubuntu... IMO, not fit for servers, not because of Debian, but because of the current disregard for many of the "standards and things" long standing issues imo. Mint is in a similar boat, but because of Cling to old ways and workarounds to make it happen. Official supprt is sometimes splotchy or inconsistent. It is also moderately Spendy.

I'd stay away from Arch, Gentoo... period. To much of "tuner" distros. There are others like them. To much in variances to make them Commercially supportable, easily enough.

Debian... always gets a thumbs up from me. Most Commercial Software works with it, once you thump it.

In general, VAR support is important.

RHEL/CentOS/Unbreakable are effectively the SAME distro. Huge lists of VAR and Commercial Program support.

Ubuntu/Mint also have large lists... though I'd go with Ubuntu Server before Mint.

SuSE, just a well rounded and very well supported product and distro.

Gentoo and the like (Arch, LFS, etc...) there a few and far between VAR/Commercial entities that appreciate them and support them. Far worse support than Debian, but again you can usually get everything to run on them. With any "bug" or Feature problem... replication of problem on a "supported" platform is typically required.

Debian... you know how I feel about Debian. It is my go to for everything if I'm allowed. VAR support typically isn't quite as large as the others, but funnily, I can get most things for CentOS and Ubuntu to run on Debian without much effort. Even programs and add-ons that specifically say they *CAN'T* work on Debian... usually it is a way to minimize support issues. Though, if you have a Bug, you have to replicate it on a Supported distro. So, you might be stuck.

There is my opinion... take what you want and throw out the rest.