Tileset 7 is just too complicated to be used well, I think.
I've seen the twirly things from 8 used a few times, but never significantly.
11 is very interesting but I've never seen it used (that I recall.)
I've seen the pillars and platforms (and I think the stained glass) from 12 used well, but the little stairs (probably the most fascinating part of the tileset), like you said, simply don't match up, and as such are basically unusable, unless you don't care at all about SGEs (as I have found myself, frustratingly).
The pillar/bar things in the upper right of 14 were used in the level Seashell by Jigganis (http://knyttlevels.com/levels/Jigganis%20-%20The%20Seashellv9000.1.knytt.bin), reasonably effectively. However, the holes in them are actually white instead of transparent, making them look very out-of-place (except presumably somewhere with a pure white background).
The wooden framework thing that takes up nearly half of 15 is just too complex to use well. (I have tried.) The tileset's name implies that it's meant to be a bridge, but the inconsistent division of the framework into tiles makes it incredibly awkward. The solid ground-stuff in the middle has been used a few times, though.
The eyes from 16 have been used to great effect in the Jolio series by Noi, but the rest of the tileset hasn't. The weird dripping thing in the lower-right is obviously a set of frames for an animation, but it was made before custom objects, so it's never been used to my knowledge. (Using shifts to make it animate would be unwieldy, and too awkward to use outside very special situations.)
Parts of 17 have been used, but when it's contrasted with other, more "normal" tilesets, it just doesn't work.
18 has been used quite often, but parts of it (the weird thing in the upper right, the rectangle-dome-structure-thing in the bottom center) have never been used.
20 is very pretty, but its perspective is inconsistent, making it awkward to use. (Though I've seen the non-isometric bits used a few times.)
23 is just too specialized, and I've never seen it used.
Tileset 32 is also very pretty, and has only ever been used well that I know of once, in AA's/ITA84's Temp (http://nifflas.ni2.se/forum/index.php?topic=339.0).
34 gets used pretty often.
I've only ever seen 37 used seriously once, in The Lighthouse by El Gabe. (No link, unfortunately.)
I think 38 gets used quite a bit in a part of The Explore Challenge.
I've only ever seen the "portal" bit in the upper-mid-right of 39 used, from that tileset.
40 I've never seen used.
41 gets used often.
42 is another one that's so weird/specialized it's never been used. (Though I believe that there's a Knytt Experiment level by Nurykabe which uses a very similar tileset, if not the same one.)