I could not play some of these, especially the earlier ones. But hey, I thought this would be a neat topic, so now you must bear my babbling. I will mostly talk about the quality of the ports, not the games ported, but it depends case by case.
Sonic the Hedgehog, Sonic the Hedgehog 2 and Sonic Spinball released on iOS in 2009 will be ignored due to being emulation, not ports. Sonic the Hedgehog 4 Episode I and II will also be ignored due to being originally intended for mobile.
I will dedicate another article to original mobile games as there is very interesting stuff there too.
Oh, starting with a hard one. In the early 2000s, Sega launched a mobile service in Japan called Sonic Cafe. It is a subscription service that ran from 2001-2007 and provided original Sonic games and mobile ports for NTT DoCoMo's i-appli phones. Most of the games were just puzzle and card games though, and only some of the games made it to the United States and Europe via the Sega Mobile service. Because of the very specific circumstances needed to own some of these games, tha majority are lost media besides some screenshots. In this page you can find a list of whether or not a Sonic Cafe game is lost. Only two are found at the time of writing this article, that should say volumes about their rarity.
Unsurprisingly, this one is lost too. I can only judge it from screenshots and descriptions originating from the now defunct Sonic Cafe website. Have a crude translation of the page I did with a translation service. If you speak Japanese and can offer a better translation, please contact me.
As you can tell, the images do not really tell me much about the game, and the website's description can describe almost any Sonic game ever. It doesn't look very good though, the level design seems rather empty. Of course, the hardware is extremely limited, so I doubt they were able to do much. Keeping my hopes up this gets found one day. This is not really a port, more of an original game, but I realized this too far into writing it so you will get it anyways.
Have you heard about the Nokia N-Gage? I'd assume people who spend even a fraction of their time here would have at least heard about it, but here is a quick summary. With the boom of handheld consoles, or just the Gameboy line really, Nokia saw a fresh market to make some serious cash. Since they made phones, a handheld console shouldn't be that hard? Except they were cowards and didn't commit fully to the concept, making some weird phone/Gameboy Advance hybrid that has a design reminiscent of goatse.
Anyways, it failed beautifully as expected, but they had Sonic N as one of their launch titles, and that counts as a mobile port. Despite the name, it's just a port of Sonic Advance for the GBA. Except, the N-Gage screen ratio was really not suited for gaming. Imagine Sonic Advance but you play as the character in a very narrow corridor, severely undermining the gameplay, with ugly borders covering the screen as a fun bonus. The framerate constantly chugs and the sound quality is significantly worse. If you want to play a worse Sonic Advance and you're one of the freaks (positive connotation) who own an N-Gage, this is for you.
This is another one of the Sega Cafe games, though this one was released in the west. This pretty much just a Java port of Sonic 1 with some quirks. I could not find a rom of the game (a source I was reading said a dump exists but did not provide a link, so inform me if you found anything) and only found some low quality footage of the Verizon version, so I'm relying on the Sonic Retro description of the differences.
This is okay if you lived in 2005 and wanted to play Sonic 1 on the go I guess, your only other option was the godawful GBA port. The game runs pretty slowly from what I've seen in the footage, and for some reason the Verizon version specifically has a lot of sound effects missing. Of course, with tiny phone displays you get a crushed aspect ratio that makes it hard to know what comes ahead. Besides the missing sound effects, you get a midi rendition of the iconic soundtrack, that frankly doesn't sound too bad. The developpers couldn't make software that crams a Yamaha YM2612 chip in your phone, as awesome as that sounds.
Some animations are missing and some assets might differ a little in size and palette, nothing drastic or very noticeable though. Difficulty tweaks and options were added, to compensate for the added challenge of the aspect ratio I suppose. The special level is missing, entering the giant ring gives you a chaos emerald automatically. And the HUD is completely different for more readabilty. It is useless to play this version unless you are stuck in a mountain with your flip phone that is one of the specific models that were supported and happened to have a copy of the game when Sega Mobile was around. This happened once to my buddy Eric.
Yet another Sega Cafe port, and this one is also lost media. I will again use screenshots and descriptions from the defunct Sega Cafe website. Have another shitty machine translation I did.
It just looks mostly like Sonic 2 again. I'd imagine it would have a sluggish performance like the previous port given the hardware. From the screenshots, I see that the HUD takes way less screen space which is much appreciated, though the game must still suffer from the narrow aspect ratio.
This time, the descriptions given by the website actually help a lot. This port has nice extra features, such as a boss attack mode exclusive to the "Mega App" version for NTT DoCoMo's FOMA 903 series. You can also resume the game from the last level you played or start from any level you would like to. And finally, you can submit your high scores to a national leaderboard of some sorts. It is useless to play this version unless you are stuck in a cave with your Japanese flip phone that is one of the specific models that were supported and happened to have a copy of the game when Sega Cafe was around. This happened once to my buddy Eric.
Another port of Sonic 1, it is impossible to go anywhere without this game in your pocket I'd imagine. This was released for the iPod, specifically the fifth-generation iPod, iPod classic and third-generation iPod nano. Since I could not play this myself as I do not own an iPod, I'm relying on footage and Sonic Retro.
First of all, this is the first port that seems to run well, though the loading screens are quite long. Also the first one that doesn't suffer from aspect ratio problems. I don't know how easy it is to control games with the clickwheel, Sonic Retro the description of the video footage seems to support that it is not. You can control the game with the touch screen as an alternative option but it seems to be worse. The music is just recordings of the Genesis/MegaDrive soundtrack, so it is completely faithful. The extra features are an auto-save function, infinite continues and a tutorial, much needed to make the control scheme less painful. It is a good port, but not on a good device for gaming.
Unfortunately I have to cut this article short for the moment, a lot of work and research went into it and it has become too tiring and time consuming to finish quickly. Due to life events I won't be doing weekly articles for a short time (the last one has been a little while ago as you would have probably noticed), so have a part of this one early.