Android boxes are supposed to be cheap. However usability won't really be there as much.
Like Ned said, you want an x86 (desktop/windows) processor type.
You're specifying multiple different and conflicting criteria. You're not worried about price, but are looking at a 45 dollar raspberry pi. It's very confusing. You need a feature list of what you want XBMC to do before anyone can help you. Without an idea of size, features, etc. we have no clue.
Because of the fact that you Specified you wanted the BEST and that you were NOT worried about Price, These are my recommendations
you'll need to shop further in each class if you're looking for the best DEAL but all of these are decent deals given the price range:
Laptop: ($300-$400)
http://www.dell.com/us/p/inspiron-15-352...on-15-3521
Processor 3rd Generation Intel® Core™ i3-3227U processor (3M Cache, 1.9 GHz)
Operating System Windows 8, 64-bit, English
Display 15.6 HD (720p) WLED with Truelife (1366x768)
Memory2 4GB3 Single Channel DDR3 SDRAM at 1600MHz
Hard Drive 500GB 5400 RPM Hard Drive
Optical Drive 8X CD / DVD Burner (Dual Layer DVD+/-R Drive)
Video Card Intel® HD Graphics
Warranty 90 Days Premium Phone Support + 1 Year In-Home Service after Remote Diagnosis
Weight 4.96 lbs
I like using a laptop for XBMC hooked up to a TV. You get XBMC, but you still get to be able to take the laptop with you on the go if you actually need it. Also, I find it easier to trouble shoot. You can take it with you to a different router, location to see why internet/streaming isn't working or something.
Desktop ($300-400)
http://www.dell.com/us/p/inspiron-660s/p...piron-660s
Processor Intel® Pentium® processor G2020 (3M Cache, 2.9 GHz)
Operating System Windows 8, 64-bit, English
Memory1 6GB2 DDR3 SDRAM at 1600MHz
Hard Drive 1TB 7200 RPM SATA Hard Drive
Video Card Intel® HD Integrated Graphics
Warranty 90 days Premium Phone Support + 1 Year In-Home Service after Remote Diagnosis
Nothing beats the power of a full fledged Desktop. Lets be real, you can tuck away just about ANYTHING. You don't need an extremely tiny PC that you pay extra to have a slow processor and less ram. Why not have it all? You get the expanadability allowing you to add whatever you want inside, the ability to load ANY skin without worrying whether it will eat your processor up, and it will be super fast/responsive. All at the same price point as a laptop or small mini PC like the NUC. Since your HTPC is almost ALWAYS hidden why not?
Mini PC (I choose the NUC!) - $400
http://www.newegg.com/Product/Product.as...6856102002
Intel Core i3 3217U 1.8GHz
This is a little more work. You have to buy the RAM, SSD (hard drive), separately as well as now how to put them into the device. It really isn't as hard as it looks though. It's EXTREMELY tiny at 4 inches by 4 inches and is still fast enough to do everything you want. However, if you plan to put movies on here (rather than stream from a separate server PC) you won't be able to fit as much. Still though, if you NEED it to be tiny because you don't have a place to tuck a computer away, then this is the route to go. You can NEVER go wrong with intel processors (hence why all choices I recommended are). Since 2008, Intel stopped fooling around with the Pentium 4 and went straight for the jugular with their processors.
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After this you have devices like the Rasp Pi, or an Android Device. Really not my recommendations though as I don't know much about them, they require more knowledge, and aren't completely polished. I dunno much about OpenELEC either since I'm a windows person and I like to still have the option of tabbing out of XBMC to possibly use my PC for other functions if I have to/want to. So I won't really elaborate on these options, especially when you say cost isn't an issue and you're looking for the best you can buy. This is the best, quite simply. You can of course look for other Desktops/Laptops that fit the price range but this gives you an idea of the price/power I'd personally look for when looking for the BEST. Avadirect.com and other sites like that other Nonbranded PCs that will offer even better performance as you're now not paying a premium for a brand name.
Hope this helps.