What a seemingly simple yet complex question!
There are basically two versions of Windows 8 for tablets. There is the RT version and the standard version. The RT version only runs Windows 8 apps, similar to the iPad and Android tablets. The tablets that run standard Windows 8 can run anything your normal desktop can (minimum system requirements notwithstanding).
I am not even bothering to look at the RT tablets. In order to have a tablet device that will truly do “real work” you need one running the full Windows 8 version.
The next thing to consider is what you will be doing on the device to determine how powerful of a tablet you need. Of the tablets running real windows, there are the Atom processors and the Core i5 processor like you would have in a regular desktop. There are pros and cons to both. The Atom processor is lower power and is only 32 bit so will only support up to 3GB of ram (most of these are coming with 2GB), but has better battery life and will run most programs like Office just fine. The i5 is a standard desktop processor, it is 64 bit and will support more than 2GB of ram (most of these are coming with 4GB), but has lower battery life (more on par with a laptop than an iPad). Both varieties generally come with detachable keyboards. The other big differentiator is that the Atom systems are coming with 64GB of storage space and the i5’s are coming with 120GB.
This next part is purely my opinion.
I’m looking to get one of the higher end i5 systems. Since I am looking to get something sooner rather than later, my options are Samsung or Lenovo (there are others, but I like these guys best for quality). If I want a Lenovo, I have to get the Thinkpad Twist. I am not as fond of this one because it is not a true tablet, but a convertible ultrabook. The screen does not detach from the base keyboard, but folds over to use as a tablet. There is nothing inherently wrong with this, but it does add thickness and weight when I don’t necessarily need that all the time. So, that leaves the Samsung Ativ Pro 700T. It is a true tablet that has a keyboard dock, meets all my specification needs and is available now. One other thing I realized about the Samsung tablet was that they have been making the top rated Windows 7 tablet for years, so they already know what they are doing in this particular arena.
All that being said I am hearing very good reviews on the Surface RT tablet that is out now and I would expect that to not change with the Surface Pro, but I do not have complete specs on what that one will look like yet.