Answering a question HERE, I've thoroughly re-tested the current version (1.5.3) of AnyPlayerHD and found out that it's equally bad (very low-performance decoders, no hardware MKV acceleration etc. etc.) as was the one I reviewed HERE, in late May. That is: this player is still not worth bothering with as there are much-much better players - even for free (for example, iMedia Player introduced below). Stay away!
2, EC Player
EC Player, which was one of the first (article) MKV players with (semi-)hardware decoding, hasn't been updated for almost half a year. The new version has just been released with some bugfixes and the bad news: the removal of AC-3 support. This is (also) annotated in the next screenshot:
(as with all images in this article, click for an enlargened, high-quality version!)
Unfortunately, the player itself is still pretty mediocre and unreliable. (Besides, it doesn't support the wide screen of the new iPhone / iPt models.) Still not recommended.
3, iMedia Player
I've tested iMedia Player, which has just received (see the other annotation in the above screenshot) iPhone 5 widescreen support. I pretty much liked it in my original review. I stand by what I said then: it's one of the best players, decoder performance-wise. On the A6 CPU of the iPhone 5, even 1080p MKV playback is, for the most time, perfect, apart from the low resolution video also visible in the following screenshot (playing back the standardized Monsters test MKV):
Interestingly, the highly recommended (get and backup it before AC-3 is removed from it too! The current, 3.7.1 version still supports AC-3) It's Playing also plays back the same video pixelizated (in, as opposed to iMedia Player's purely "brute-force" software decoding, hardware decoding mode):
All in all, iMedia Player, as a free(!) player, is highly recommended unless you want to use it for, say, MTS playback, which it's still equally incapable of. (Haven't tested the other “cons” in my original review, only the lack of MTS support. Therefore, I don't know whether they have been fixed or not.)
4, HD Player Pro and yaPlayer
I've recommended both players in their reviews (that of yaPlayer HERE). Some days ago, shortly after publishing my latest review (also pointing out that HD Player Pro “left” in AC-3 support in their player), they both have been removed from the AppStore (because of their AC-3 support, I bet) and haven't been reintroduced since. I don't know what the future holds for these apps – after all, AVPlayer(HD) has been absent for almost two months and it's still unknown when it reappears in the store.
UPDATE (about a day later): 1.) regarding the last screenshot I've presented, I've found out why It's Playing plays back hi-res videos using non-Retina low resolution on the iPhone 5 (or any other, small-screened but Retina iDevices): you must manually tap the "Info" icon (after selecting it, bottom right corner) of each video you play back set Video > Playback quality to "Best" from the default "Automatic". (On the iPad 3/4, this needs not be done: there, the default "Automatic" setting results in high-res, Retina-friendly playback.) Doing this for HW-assisted MKV playback, no difference in playback speed can be spotted on the (very fast) iPhone 5 and is, therefore, always recommended.
This is the setting you need to set (see my annotation):
2.) Elaborating on the SSA/AAS subtitle compatibility HERE, I've thoroughly tested HD Player Pro on the iPhone 5. The player, as has often been mentioned in my articles, has excellent support for SSA. It's fully iPhone 5-friendly, makes use of the entire screen area and will also play back HW-decodable MKV's (not 10bit h24 ones, unfortunately) using hardware acceleration.
Note that it has still not been allowed back into the AppStore. I don't know if and when it is - if at all.
UPDATE (two days later): 1.) unfortunately, the latest version (1.1) of iMedia Player no longer supports AC-3. Too bad they haven't warned the updaters at all (see the above changelist). Nevertheless, earlier (pre-1.1) versions have had equally great codec support - no wonder I heartily recommended it back then. That is, if you do need AC-3, you can safely use the pre-1.1 versions. Make sure you do back up!
2.) The excellent HD Player Pro has just been let back to the AppStore. As you can see in the next iPhone 5 screenshot, the new, 5.0.3 version has dropped AC-3. If you do need AC-3 support, do not update or backup the old version first!
The list also shows "1080p MKV Player" and "playable", the latter having exactly the same update list as the former. It seems they have been further forced by Dolby to remove every bit of AC-3 support - after all, both players have been updated with (at least partly) removed AC-3 support earlier.
3.) Dolby's forcing devs to drop all kinds of Dolby support from their players (and Apple's removing players whose devs aren't obeying) is pretty much frowned upon in the advanced iOS user community. The posts in the last one-two weeks in THIS and THIS threads are worth checking out.