This all black tee is from Spencers' Gifts in 2017.
It's pretty standard stuff: an all black tee with Arms-cross classic Sonic just standing there. He's sort of 'over' his vertaically written name down one side. The colors here are slightly washed-out, but it does not appear to have 'faux wear' / weathering to the design. This is in adult sizes only, so it should fit anyone. It is probably 17.99 or so and should be online & in stores too. It's an ok tee if you don't have several other that are quite similar. It doesn't bring anything new to the table, but it is a solid/ordinary design.

This shirt is called the "Green Spatter Sonic Tee".
However, it's more like the 'swirl' Sonic tee because that's what's going on in the graphic. Classic 'ready to fight' Sonic is surrounded by swirls of white...wind? Energy? It's something. The shirt itself is black, but has tiny dots of white or gray in the fabric. His name is written in a 'messy look' green font at the top. (This is probably the title's 'spatter') This is a Sears shirt from the summer 2017 line.

This shirt is called the "Gray Sonic & Tails Title Tee".
It's from JC Penny stores in the mens' section, &, it's name makes sense. It's a mottled gray shirt with the title screen winged-ring & banner from Sonic 2. It has Sonic & Tails doing their pose as they pop up through the ring. Sadly, the shirt does feature 'faux wear' weathering on the design, grinding it down especially near the top. This should be a spring/summer 2017 item, but only at JC Penny stores.

Here's a men's adult blue tee shirt for fall 2017. It has neo-classic standing Sonic in a blue Green Hill themed square for the background. The classic logo is at the top & Eggman is in the background in his egg-o-matic. What's interesting is that he's not really doing anything. It just appears he's going along on his way somewhere, ignoring Sonic in the foreground. This makes the design a little funny. The neoclassic art keeps it a bit fresh as well.
Note that the black cloth collar tag has 'Sonic' branding of some sort on it as well. But where can you find this tee? Is it a store exclusive?

Another Tee for Sonic Mania!
And it's also in mens' sizes so it should fit anyone. This particular tee is a mid-tone green, but if you look at the inset down there, you can see that it also comes in black, light blue, charcoal and white. It has 12 different sprites from Mania, done in the 16 bit pixel style. Most of them are very similar/the same as sprites already seen in other games, however the 'holding chili dog' is new/exclusive to Mania.
It's pretty plain, and other designs of the past have been a bit similar, however with 12 sprites & including the new Mania one, plus the added roster of colors to choose from, it's still an all right shirt if you didn't have something similar.

This T-shirt is called "Sonic Pals".
That's an appropriate title, as the light blue men's tee has neoclassic art for Sonic & Tails in a simple graphic ring. It's jumping Tails & leaping/profile Sonic. The bottom is a large, classic ful color logo. The neoclassic art & simply drawn ring keep this shirt looking fresh. With true color on the characters & logo, the large design is colorful & bright. Notice the lack of weathering, it's a good, crisp design.
Look at the inset: the tee comes in black, white, mottled gray, charcoal, and off-white as well. This may be an Amazon only item & it is new in Fall 2017.

This tee is called "Fist Bump Sonic Tee".
It's a fairly appropriate title...if Sonic was waiting for a fist bump. This is a neo-classic art of him, where he's just kind of standing there with a fist up a bit and a hand on hip. The classic logo is above him, rather large. The blue for the letters is actually the blue of the shirt, left to show through the design. This is a mens' sizes tee, so it should fit anyone. Notice the inset too, it also comes in dark blue & charcoal gray/sort of mottled.
It's a rather plain and ordinary design with a simple Sonic. It's nice if you don't already have something similar & were looking for something that isn't too busy. This may be an Amazon only item & it is new in Fall 2017.

This is called the "Group Shot Tee".
It's appropriately named, as it has all of the classic Sonic characters in some fun neoclassic art poses. You've got leaning Eggman in the back, a profile of Amy Rose, gesturing Sonic, waving Tails & fists up Knuckles. At the top is the classic logo in its original color. The tee itself is white, and new for the end of 2017.
It MAY have that 'faux wear' or weathering for the design, as it seems a little pale/washed out looking. Which is too bad...because this is a great tee for Eggfans or Amy fans since they seldom appear on tees, especially in classic style like this. Still, it's an unusual enough tee & colorful too, with everyone there. It's in men's sizes, so it should be good for anyone. How much does it cost?

This shirt is called the "Running Sonic Gray".
It has a rather boring name (that's likely been used before) but it does feature neoclassic art, to at least make an attempt at freshness. The shirt itself is a plain medium gray. It has the classic logo large and in full color at the top, with the sort of falling/maybe running...well it's an action pose for classic Sonic right under it. The logo itself looks a bit 'washed out' in color, but the shirt doesn't appear to use faux weathering. You can see the black cloth tag in the collar has the Sonic logo on it as well, but where is this available? It was one of the last few releases for 2017, and is a mens' size so it should fit anyone.

Even more Mania?
Sega Shop seems to think so. Here are 4 new Sonic Mania tees, each with the 'box' graphic/a variant of it. They come in 2 colors: dark blue or black. The first two have just jumping Sonic with the 'colorful shapes' circles

for the background, and the other 2 have Sonic Tails & Knuckles on the same background. The second shirts also feature a smaller Mania logo. These are probably a 2018 item, and may be either Amazon or Sega Shop, and should also be sized for adults so they'll fit anyone.