As someone with a pretty large classic games collection, my answer is 'so far, I don't really want any of them'.
For me, unreleased games are one of the few things that might get me to get something like these someday. On this front, of the three, the most interesting is the SNES, because it has the previously unreleased Star Fox 2. The two unreleased games on the Genesis Mini, Tetris and Darius, aren't nearly as interesting (both are on lots of other systems after all), and the TG16 Mini has no unreleased games. So on the unreleased games front, I'd say SNES > Genesis, TG16 didn't compete.
As for import games -- putting games from other regions on the thing, without making you buy one from another region to get some -- the SNES has none, and even if you get the Japanese one, there isn't a very different library. The Genesis has Monster World IV and Mega Man: The Wily Wars, which are nice inclusions, though MWIV in English did previously see release on the Wii, X360, and PS3 as a download and Mega Man TWW had Japanese and European physical-cart releases back then. The Genesis/MD Minis locks most other-region games to the other versions of the thing, unfortunately, so you need to hack it or buy three of them to get most of the import games -- this is a definite negative! Like, that list of games is pretty good, apart from the omission of Sega's best game ever (Sonic 3 & Knuckles), but it's way too broken up across regions, I'd never buy three of them. As for the Turbo/PCE Mini, it's very cool that apparently all of the games will be in all of the systems, except for that one game which is exclusive to the white model (Tokimeki Memorial), that's great. If that is indeed how it works, on imports the TG16 Mini wins easily since there are a bunch of them and you actually get them without needing to import other region mini consoles to get them.
And last, what about the game selections in general? Well, the SNES Mini has a small but focused library of many of the consoles' best and most popular games. For overall quality and best representing the console the games came from, the SNES Mini is probably the best of this batch, though it is unfortunate that some games are only on each of the two models -- why no Tetris Attack in the US one, for example? Some genres are totally missing as well, such as shmups (and there are some great SNES shmups!). Still, it is a good lineup. Nintendo got key third party support for many of the top third-party games on the console.
The Genesis Mini has a very good library as well, but it's held back by lots of games being exclusive to various regions and the consoles' best game being absent. Sega also got solid third-party support from a bunch of publishers, which is great, but while Nintendo has a few key games only on each regions' version, Sega has way too many.
As for the Turbografx Mini, it's an interesting library. I'm not sure what to think... on the one hand it's a pretty good lineup with lots of good games. On the other hand, it is heavily weighted to Hudson and Konami games. Now, in the US, NEC or TTI published almost all games, and the US games in the TG16 Mini are all NEC or Working Designs (their two HuCard Taito ports only, not any of the CD games)-published titles. Makes sense. The NEC US-published titles are mostly Hudson games (including Hudson's ports of Falcom's Ys I & II and Irem's R-Type), but there are also one each from NEC, Irem, and NCS Masaya (Space Harrier (licensed from Sega), Ninja Spirit, Moto Roader). As for the Japanese library, it's mostly Konami and Hudson releases (including Hudson's ports of Tecmo's Ninja Gaiden and Falcom's Ys I & II). The exceptions -- NEC's ports of the Capcom game Ghouls & Ghosts and the Sega game Fantasy Zone; NCS Masaya's Cho Aniki; and Taito's Super Darius. It's a very good lineup but it could be a bit better -- I'd have liked to see some of NEC's original titles, for example -- how about Metamor Jupiter, Magicoal, Gotzendeiner, and such? Some of Falcom's own games would have been great too, most notably the two Legend of Xanadu games. Etc.
As for my own personal thoughts on the game choices, looking at my 'favorite games' list, 10 of my top 20 (and ~6-10 more of my next 50) TG16/PCE/CD games are in the TG16 Mini. The US Genesis Mini has 7 of my top 15 (and 12 of my next 50); two more is only in the Asia version, but that version doesn't have two of the ones on my list the us one does have. SNES Classic has 7 of my top 15. There's one more is in the jp version, but it loses a game from my list. (and like 4 of my next 50, again with one more and one less in Japan). So, the SNES has the fewest games I really like. On the other hand, all of the SNES games are good games, while the other two use spots on mediocre to poor games, so it's close.
In closing, I'll continue to stick with my physical-games collection for these platforms, they're three of the best consoles ever and while these devices are nice, they never include all of the games I want, because they can't. And if you're just buying them to hack them and put roms on them, there's no reason to buy a mini console for that, you can do that almost anywhere. I'd rather play the real games anyway. But for people buying these because they look cool and get you legit copies of classic games, if I was going to recommend one of these based on the libraries, if the emulation is good -- and this is definitely a big if, we have no idea yet -- maybe get the Turbografx one, since it's less likely that people have played those games than the ones in the Genesis or SNES mini systems and the system is very close to as great as the SNES and Genesis are.