The most desirable is the official Sinclair Spectrum Next
The most desirable is the Sinclair Spectrum Next, which is fully licensed and legal and officially a Sinclair licensed by the original company (made by SpecNext Ltd.).
The Next is the most compatible of all Spectrum platforms, being able to switch mode between all the various official Spectrum releases and most clones, to give an authentic experience with near perfect timing. And it can do so with output to HDMI which with regard to border effects designed for CRT technology with different timings than modern screens is a mighty feat (and of course it does support CRT with original timings).
It also has a bunch of peripherals baked into the core, providing 512 colours, 3 (or arguably more) layers, sprites, 4 AYs, multi-face type functionality, 2MB of bankable RAM, modern display output, SD storage (a Spectrum with a hard drive!), Wi-Fi (download and play almost any game in a few seconds), hardware compatibility via classic edge connector, and the nicest keyboard known to man (modern membrane with modern performance), although some may prefer the mechanical keyboard of the N-Go. This might not interest those who just want to play manic miner forever, but if you like to tinker or have unfulfilled childhood dreams, it's really, really, great, and even as a game player, there are now hundreds of titles to explore created both commercially and as passion projects by newbies and established giants of the era. It's an active place for innovation in an accessible bare metal environment. I genuinely haven't turned my Xbox on for at least 6 months.
The N-Go is a clone of the first release of the Spectrum Next (with a couple of fixes) and 2MB RAM. The original Next only had 1MB with sockets for the £10 RAM to upgrade it to 2MB which most people did.
The second release has 2MB and a larger FPGA than the N-GO so although both can run the QL and other cores, the KS2 can run a better version with sprites etc. Although there many community cores for the KS1 or NGO which have not yet been ported to the new larger FPGA.
The third release (KS3), due to deliver in the next couple of months, has an even larger FPGA, allowing for a full featured Amiga implementation, and 4MB of RAM for alternative cores (the original Spectrum Next core specification is 2MB and will remain so for compatibility, now there's hundreds of titles taking advantage of the new capabilities). This third release includes official cores for the C64 and Amstrad CPC. All official cores aspire to provide full VGA and HDMI output and SD support, although community cores tend to be much more MiSTer like with varying implementations. The official cores will include key combos for platform unique keys not easily mappable to Next keys.
There is an emulator for cross development, but the only real way to experience it properly is to get a Next, N-go, or XBerry Pie (£100 cheapest hw option).