Bitten by the sidescroller bug...

[Xion]

You guys are very difficult to please. I like it.
So, here's the latest version, trying to please everyone:

How's that for an attempt?

This one was my favorite. The newer, lighter ones look bubbly. Bubbly in a different way than this one. Bubbly in the kind of way kids blow clouds of bubbles, only a little more violent and with a bang.

[CaKsTeR]

Agree with Xion, the newest version looks like bubbles dissipitating into nothing. I think you were better off with the one previous to it.

I'd criticize more, except that I can't find anything wrong with any of the animations. I'll leave the nit-picking to the pros 

[.TakaM]

These ones almost look like they belong underwater.

I think these are the best ones.

Keep up the amazing work btw.

edit-
Consider this a vote for #2, the translucent smoke shows more depth and ultimately fades away the smoothest, I think #3's smoke is too solid and would look rather distracting in game.
But to be honest, any of them would be fine

[twodayslate]

These ones almost look like they belong underwater.

Keep up the amazing work btw.

Could not agree more!

[Howard Day]

Wow. Okay, seems like everyone has a different take on how these things should look. Well, I'm putting it to a vote.

1:
2:
3:
4:

And if there's no clear winner, then I most likely will just choose one. Right now I'm pretty much leaning towards #3. In the mean time, I'm going to lave it alone for a good long while. I'm going a bit insane doing all these variations.

[9_6]

Definitely 3.
Smoke doesn't reflect light like it happens in the other variations imo.

[Photocopier]

3, I think the paler ones just look unrealistic.

[Crow]

Out of the four, I'd say three as well. I believe that the highlights on the "earlier" smoke particles are too strong though. All of the four animations have that problem. This makes the smoke particles look a bit like metal balls at one point. Not cool.

[Lizzrd]

make the smke on 3 more brown.

[CaKsTeR]

I vote 3, looks the best out of all of them