If you're going to use a Sid based system, then, to my way of thinking, there is no better one than Sid itself. However, if you are starting fresh from scratch, I'd pick antiX core as the one that gives you a few more tools to work with, and siduction, by a hair, over Semplice, in terms of complete, prepared, ready to go systems. For all of them, I like adding the smxi tool. Yes, I can configure them all without smxi, but smxi just makes the task faster and simpler, regardless of what aptosid and siduction developers say about using the free stuff only and sticking with the core system. If we want to do that, then forget them: use just Debian Sid!
So in the Debian Sid world, Debian Sid and antiX core, for me, get the nod, but I have to say, working with Semplice today, I've pretty much (gradually) taken out their stuff and put in the stuff I use, so there is very little difference between Debian Sid, antiX core, and Semplice for me. Same goes for siduction; they're all quite good.
In the Debian Testing arena, again, what can be better than Debian Testing itself? Once the system is completely set up, no need to go elsewhere. If you are starting from scratch, however, it's hard to beat antiX base. The antiX M12.0 base edition, even though there are some packaging defects, documentation, and tools still to be polished and finished, is one of the best there is, and antiX M11.0, already released, was a great release last year. If I were starting fresh (which I did recently, I would not hesitate to use antiX M12.0 Test 2 base, or any of the internal test builds that are taking place right now. But there are a few other good Debian Testing derivatives that also work quite well. One that I like, and I have installed on my Lenovo, is ZevenOS. I have Version 2.0, the "Neptune Edition". No reason for me to get any newer release, because Neptune looks nice and is just as up to date as any newer releases they may have come out with since then. Starting fresh, sure, grab the latest version, but for me, "Neptune" does very well.
For Debian Stable, that's one area where I think you can do better than the Debian release. SimplyMEPIS, to me, adds demonstrable value. It's much faster and easier to install, and you can choose to either stick with it, as is, and "age" it nicely, as Debian itself does, or you can, as you need them, add newer software packages from the MEPIS CR - their Community Repository. Debian Stable is great, of course, and that's where the great software comes from.
Canonical, though often criticized for not making many direct software contributions to Linux or to Debian, actually DOES provide contributions in several ways. First, the marketing that Canonical does for desktop software is something that not only Debian, but the entire Linux ecosystem has needed for years. Red Hat provides what's needed on the server side of things. SUSE has done good things on both the desktop and the server, but Canonical has made more inroads, offering four or five of their own sponsored derivatives, and they always seem to be in the news about something in free software. But something that not many people see is that they DO give back to the Debian project. There are numerous bug fixes that make their way back to either Debian Sid or Debian Testing from Canonical's efforts, and in addition to that, tools like update-manager have, over the years, seen their way to Debian, and the simplicity that Ubuntu and its derivatives have added to the installation and configuration process have slowly, but surely, made their way into many Debian projects, so that Debian itself is no longer that difficult beast to install or use, so Canonical can be thanked for their role in that.
As far as Ubuntu derivatives that I like, in the Canonical camp, my favorite by far is Xubuntu. I like it nearly as much as my Debian systems (which always end up getting Xfce installed on them). Xubuntu is Canonical's community version of Ubuntu that comes with Xfce instead of GNOME. The first Ubuntu derivative that I actually started using was Kubuntu. I don't find it quite as stable during testing as Xubuntu, but released versions are always pretty solid. For lighter computing than even Xubuntu, the LXDE-based Lubuntu has been quite useful for fast start up and use mainly when browsing is all that's planned for the login session.
Personally, I am not a huge fan of Mint and its derivatives, but there are a couple of them that are quite popular, beginning with the main Mint (GNOME-based) release. Mint, which is, in the main version, a Ubuntu derivative, has done a lot of work to deal with the erratic nature and major changes that have occurred since GNOME 3 was released. Ubuntu came up with Unity as their answer. Mint came up with a couple of alternatives, including Cinnamon and MATE, which have been popular to smooth the transition to the vastly different desktop style introduced with GNOME 3. I'm not a GNOME fan, so I usually ignore this stuff, but Mint also has a KDE edition, an Xfce edition, an LXDE edition, and others as well. But perhaps their best derivative work started as an experiment: a return to Debian rolling release testing repository package archives instead of Ubuntu derived archives. The result is called Linux Mint Debian Edition (abbreviated LXDE), and it's one of the most popular Debian derivatives, and probably second only to Mint itself within their community derivatives. Debian lovers who are also Mint lovers might want to give it a try. One of my friends who likes Debian, but struggles at times with software updates has had good success with LMDE.
That's my summary of what several of the top Debian-based distribution alternatives are out there right now. There are many other good ones in addition to these, but for general purpose use, and also for my own personal use cases, these are the ones that get the most attention from me.