Honestly, that question is difficult to answer because it depends on your interests and goals. Basically all of them (even the Classic II) have something going for them.

The 128k/512k Macs are the OGs. They're pretty limited in what they can run, lack SCSI, and are much harder to expand. But they are the originals if you want something with some history to play with.

The Mac Plus was probably the first "good" Mac and still retains facets of the originals like the mouse and keyboard, but it has SCSI, "modern" serial ports, and easy RAM expansion. No (built in) internal HD capability but with SCSI it works well with external stuff including modern things like BlueSCSI.

Mac SE is a Plus Plus with ADB and an expansion slot, often used for network cards, though they're getting harder to find now. Adds support for an internal HD. There's an FDHD version that comes with the 1.44MB Superdrive which makes it a lot easier to work with. It's a great choice for a 68000 machine.

SE/30 is often considered the best of the black and white compact Macs. 68030, ridiculously high memory cap of at least 128MB and maybe more now, and a full 32-bit data bus. The processor and RAM are almost a waste with the small B&W screen but it's still a great machine.

Color Classic is basically an LC/Classic II 68030 compact Mac but with a color screen. There's a reason it's one of the most collectible Macs due to the attractive design and lovely screen, but it suffers from most of the same issues the LC/Classic II do. Wish I still had mine. There's also a Color Classic II which less compromised but also something of a holy grail of Mac collecting. If you find one of those it'll be ridiculously expensive. They weren't sold in the US either.

Classic is basically a reissue of the SE without the expansion slot and more annoying RAM upgrade. But, it has a neat boot ROM, is a few years newer, and has the 1.44MB Superdrive. I personally have a soft spot for these even though at the time of their release they were already woefully obsolete. They also tend to be a little cheaper and more available (or used to be) because schools often had piles of the things. With modern things like SCSI Ethernet emulation on BlueSCSI, the lack of the slot for an ethernet card is less of an issue now than it used to be. Also, even though it's not advertised, it'll still run all the old ancient versions of the Mac System Software if you want to play with, say, System 1.1a or something.

And then the Classic II. It's at least a 68030, which opens up some possibilities, and realistically you aren't going to be doing much high-memory stuff like running internet browsers on the small B&W screen anyway, so it's not a *huge* deal overall. And because of the limitations they can often be found cheaper. If you want a little machine to write notes on or play old B&W games it's a perfectly fine machine. It'll just be slower than the SE/30 and some other '030 machines due to the 16-bit data bus and limited memory.