I know half of these aren't even on the service yet, but my thoughts on the GG Sonic library:
Sonic 1 is decent enough. Master System version is preferable, but the Game Gear version's fine too.
Sonic 2 is definitely a fairly solid little game, but it has a lot of bullshit dragging it down. Hang-gliding in Sky High is annoying. Pipe mazes in Scrambled Egg Zone are annoying. The total lack of rings in any and every boss fight is annoying. Plus, the first boss fight was made harder for the Game Gear version; aside from a smaller arena for you to fight it in, the bombs that drop on you (which, since the stage lacks rings, kill you in one hit) are no longer predictable. In the Master System version, they dropped with the same arc every time; some horizontal variation in placement, but easy enough. In the Game Gear version, they vary between two different arcs in addition to the horizontal variation and significantly reduced view, making the boss fight nearly impossible without inhuman skill, the ability to see into the future, uncanny amounts of luck, or save states (the last of which the 3DS VC gives you, thankfully). Master System version is recommended over the GG version, as such; you can actually see the second world.
Sonic Chaos is pretty good, although Sonic's suddenly a lot heavier and more sluggish than he was in the previous outings. Still, kinda recommended. I'm not sure which came first between the Game Gear or Master System versions, but they're pretty much identical this time around, so who cares?
Sonic Triple Trouble is even better, and yet even slower. I don't think it's particularly spectacular, but hey, recommended.
Dr. Robotnik's Mean Bean Machine is Puyo Puyo, but it has a puzzle mode that the home console ports didn't have. Might be worth a look for that alone.
Sonic Spinball is shit. Not like the Genesis version was going to win any awards, but it was at least kinda decent; this version, to my understanding, really isn't. Avoid.
Sonic Labyrinth... okay, this one's a guilty pleasure of mine. I do not recommend it, of course, but it's kind of like a Sonic minigolf game. People give it a bad rap because your walking speed is abysmal and your Spin Dash is nigh uncontrollable, and they're not wrong, but I've had fun with the game in spite of it.
Tails SkyPatrol has an interesting concept - a horizontal shmup with a boomerang shot - and it's interesting to see the villain designs they came up with, but it's not that good a game.
Tails Adventures is unlike any other Sonic game out there - a fairly slow-paced, stage-based Metroid-esque title, where you go through stages, throw bombs at ducks, unlock more stages, find power-ups, use those power-ups in old stages to unlock even more stages, go underwater in a drill sub, go in the air in a drill sub, etc., until you've beaten the game. Gets some bad rap from the press because it's not fast, but hell, I love it.
Sonic Blast is horrid. In their attempt to mimic Donkey Kong Country and its successful CG artstyle, they kind of let every other aspect of the game design fall to the wayside. As such, you've got a vaguely pretty (if incredibly dorky-looking) game that doesn't play well. The Special Stages are kinda shit, too, and I hear Blue Marine Zone has a huge, totally-underwater maze in it that's a fun time to work through. Do not recommend.
I didn't bother with either Sonic Drift; sorry to say, but I was never really feeling the Pole Position/Rad Racer/Outrun style of racing that these games are centered around. Stick with Mario Kart 7.
