SNES - Very high level of detail, I think that the SNES version has the most consistant graphics detail as a whole, though something that needs to be pointed out is that each version of the game actually has their own specific examples of stages which edge each other out (Ryu's level for instance generally has a lower level of detail on the SNES compared to the MD and is missing the moon in the background ) I'm giving the overall win to the SNES due to the fact that it seems to win out in detail in the highest number of stages.

Mega Drive - As a whole it's pretty good, though not as consistant as the SNES version. Some levels such as the aforementioned Ryu stage and Zangief's stage have more detail in the backgrounds than either of the other two versions (Zangief's stage has the properly drawn chain from the arcade original as well as signs on it's fence which are missing in the other versions) on the other hand though other stages have some certainly noticibly inferior texture work, such as those for the ground in the Sagat and Guile stages which are not up to par by comparison (see 1st picture for example) Guile's stage does however, as can be seen above have more of the background elements retained from the arcade as the couple sitting on the box in the background are missing from both other ports.

PC-Engine - Very good and quite comparable to the SNES version, one notable exception is Zangief stage, which due to the lack of parallax (more on that later) is now entirely missing the foreground fence.

SNES - Great colour use across the board and in my opinion the best of the three, there's a lot more onscreen colours than the MD version and the colour choices tend to fit better than those in both the MD and PC-Engine iterations.

Mega Drive - A very good job for the most part but it's certainly the weakest of the three versions in this aspect due to the lower number of onscreen colours. With only a few exceptions the levels look very respectable, notable low points include M. Bison's level which has a very noticible lack of colour in some areas (see 3rd pic down).

PC-Engine - Very good, better than the MD version but I think still weaker overall to the SNES as some of the colours don't fit quite as well and the game has a very slightly over saturated look to it.

SNES - Character animation is great, and there's a good amount of background animation

Mega Drive - The character animation seems just as good as the SNES version, and the backgrounds actually seem to have even more animations (This is most noticible in Chun Li and Guile's stages). Other than that this is the only version of the three with an animated continue screen.

PC-Engine - Just like the SNES version, character animation is great and there's a good amount of background animation.

SNES - Great scrolling with loads of parallax.

Mega Drive - Same as the SNES version.

PC-Engine - Great scrolling but unfortunately this version is missing all the parallax scrolling of the others.

SNES - The music in this version is more of a mixed bag than people like to make out, many of the tracks completely blow the other versions out of the water but others don't fair as well. The developers had to recreate the music from the arcades from scratch and whilst doing so changed the musical arrangements about a bit, which in many cases helped to add to the quality and richness of the sound, but in others there's a few unfortunate decisions which I feel have had a negative effect on those particular tracks. Some examples which I would consider to have had better music in the Arcade/Mega Drive versions are the main intro, Balrog's stage, Ken's stage and Sagat's stage, mainly due to the fact that many of those songs were more rhythm orientated originally and are now sadly lacking in "punch" and excitement, they focus more on melody losing their driving beats, for stages that should be tense (Balrog's and Sagat's stages are towards the end of the game).

Mega Drive - Due to the fact that the Mega Drive and the original arcade machine both had similar sound chips the music could be downgraded and directly transfered relatively easily from one to the other, thus making the Mega Drive music the most accurate to the arcade out of all these early versions (though most accurate is not always neccesarily better of course). The re-made music for the SNES version is often better, but I do feel that the drum beat sound on Mega Drive is usually superior as it's not as prominent in the SNES music and often drowned out by the melody. For the reasons I've just mentioned I feel that whether or not someone prefers one version's music over another may be more down to personal taste, I myself for instance am pretty much split down the middle between tracks in these two versions, and after a quick look through the net I find that usually it's the same tracks which are in contention (Ryu's, Balrog's etc)

PC-Engine - The music in this version is pretty good, it's quite clear and crisp but is noticibly more basic in sound than both the SNES and Mega Drive versions as there's there's simply less going on in it.

SNES - Very good sound effects and good voice samples.

Mega Drive - The sound effects are generally alright but the speech samples in particular are pretty poor, they sound quite distorted and fuzzy for the most part.

PC-Engine - I would say that the PC-Engine has superior sound effects to the MD version but inferior to the SNES, the speech samples in partivular are noticibly much higher quality and clearer here than on MD.

There's honestly not really much to set the three versions apart gameplay wise as they're all pretty close to identical in everything apart from modes.

Looking at the modes, firstly we have to take into account that the PC-Engine version is specifically a port of the arcade game "Street Fighter II - Champion Edition", whilst the SNES and Mega Drive games are actually compilations of sorts that include ports of both "Street Fighter II - Champion Edition" and "Street Fighter II - Hyper Fighting". This means that the PC-Engine version lacks the increased speed modes and the option of playing with the refined fighting engine of Hyper Fighting (Ken, Ryu, Chun Li, Blanka and possibly some others received new special moves in hyper fighting to balance them better against the stronger characters)

The Mega Drive version also has another mode included which is unique titled "Battle Mode" this is basically a winner stays on mode where each player gets to pick a team of fighters to play against each other consecutively, it's quite a fun mode and due to the fact that the Mega Drive is pretty much identical to the SNES in gameplay but has an extra mode on top of it i'm going to give MD the gameplay section.

SNES - The intro is missing, but the character biographies are still present.

Mega Drive - The intro is here, as well as the character biographies

PC-Engine - No intro or character biographies

The stock controllers for the Mega Drive and PC-Engine versions are not really suited to playing this game properly (though the SNES controller is not perfect itself to be honest) however, great 6 button pads are available for all three of the consoles so it's not really much of an issue in the long run.