I call the Hellhounds (if those are the splitting missiles with mouths?) Cluster Carls (or Clusterf--k Carls after they killed you about 50 times in the same level) in memento of Bullet Bills and stuff.

Also I don't see how these add much. Sure, low-death is a challenge, but we already have the 0 death achievements for entire chapters.

Loved all of them, very interesting concepts, but you could remove the evelating platforms in the one level with the long shaft. it's more challenging with just walljumps imo.
Oh, and in the 2nd or 3d level, it's very easy to walljump between the saws to bandage girl instead of using the moving fans.

In the 3rd level, did you jump off the 1 block ledge on the 2nd platform, or did you jump to the ship first? I ask because if you did the former, you deserve that A+ time. I wouldn't have thought of that!

I did walljump off the 3x1 sized little 2nd platform, but I just found out that it's doable by walljumping to the ship as well. The first method is slightly harder to pull off, though.

I like them all, but cut back on the 45 degree slopes. The jumping mechanics on them are highly annoying, especially if you need to hit a walljump off a wall right next to the slope.
Also the last level seems way easier than some previous ones. Even with the bandage.

I have two small but incredibly interesting additions to your list.

The first one is Nikujin.
It's difficult to explain what Nikujin truly is, but it's a tiny fast-paced platformer game with brilliant music and levels that get very difficult. Perfect for an SMB forum.

The second one is Bunny Must Die: a hilariously hectic, unique and quite difficult indie Metroidvania game with time hax and other interesting gimmicks. As the title implies, you WILL die a lot. Unfortunately, it's made in Japanese only, but you can patch it if you use the translation patch.

Download here:
http://megaupload.com/?d=LTHM73E2
Translation patch here:
http://radicalr.pestermom.com/status.html

This has got to be the best present ever.

Loved all of them, very interesting concepts, but you could remove the evelating platforms in the one level with the long shaft. it's more challenging with just walljumps imo.
Oh, and in the 2nd or 3d level, it's very easy to walljump between the saws to bandage girl instead of using the moving fans.

Played this and liked it a lot. A few things to sit and think about:
Rig is perfect, but why is there a platform floating around at the middle right? I never used it.
SHAFT is fine, but the final part is unnecessarily long. It would be better with just 3 rotating saws rather than 5, I think.
B0X.0 is good, but not nearly as difficulty as some other ones. Maybe move it up earlier in the pack?
HALL is the one level I really am not a big fan of. I love the bandage location though.

I don't care much for scores, because once you got the skill and the pattern down, you can redo the levels as often as you want to get your best time again. I wouldn't want to do this for the ridiculously long levels and the harder warp zones, though, but if this gets rid of all the Naija wall skipping and 0.00s then I'm fine with it.
(Inb4 someone defending Naija wall skipping: it's not lag, SHE ISN'T SUPPOSED TO GO THROUGH WALLS)

Need to beat 2 more levels, and I'm around 1200-1300 right now I think. I also still need to speedrun most of the Dark Worlds, so yeah, there's lots of improvement coming up soon.

I'm gonna go ahead and vote now that I finally completed all the levels in the poll, lol. (can't seem to complete 7-19x, that level is still bugged as hell for me).
I'm gonna say the Kid 2. I didn't mind the Kid 1 that much, but the Kid 2 is the breaking point. It made me want to tear my hair out, but I really didn't want to redo Kid 1, so I just kept playing. Eventually I got it (and Kid 3 was a breeze fortunately) but...ugh.

I believe I got about 10 top 50 scores and maybe 20 top 100 scores, but I can't find them right now lol.  86th on In Line seems to be one of the better ones.