Favourite
Tarstuff is by far my favourite of all elements. Technically that would be three elements, that are also intrinsically linked to mothers and babies, but they are similar enough to count as one, and can be even switched into each other with tokens.
I think tarstuff has some of the richest puzzle potential of all elements. Not only that, but the three types seem to complement each other perfectly, each having their own unique properties. Also I'm a complete sucker for the the fascinating mathematical theory behind tarstuff removability. To me the tarstuffs form the holy trinity of deep puzzle design.
Best design
While tarstuff is incredibly well designed, I feel it is almost accidental how well it turned out. Many intresting properties were only discovered later, and mud and gel were more logical extensions to tar than completely new and innovative concepts. Granted, I think exactly the right decisions were made on some details of mud and gel to make them work most effectively.
However, when it comes to best original design, the aumtlich comes to mind. They still stand out to me as the most original element that got added in TCB – their ability to affect the player at a distance was completely novel. They provide a good mix of concepts familiar from the pre-TCB era as well as completely new mechanics for their beams. And they have shown to be remarkably useful in architecture in all sorts of unexpected situations. Of all the monsters added throughout the years, the aumtlich was for me the last truly game changing monster.
An honourable mention goes to the new weapons added in TSS.
Underappreciated
There are several elements that are underused, that I feel are not that interesting or well designed in the first place. I can't blame architects eschewing builders because of their chaotic behaviour. Another example is arrow rotators, which are not really flawed, but many of their uses are already fairly well covered by doors.
However, For underused puzzle element I'm making a case for a slightly unusual element: broken walls. Yea, these have been around since the very beginning. Yet they mostly serve as a basic element, that is either used to obscure secrets (like its cousin, the secret wall), or plays the role of a single use door. But as a major puzzle element it occurs fairly rarely. I really like it though, when architects find a proper, creative puzzle use for broken walls, and I have made a fair number of rooms with broken wall puzzles myself.
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Holds:
An Architects Audition,
Artful Architecture,
Salamander,
Elusive Exhibitions,
Leftover Levels,
Six Times Six
Collaborative:
Way Forward,
Advanced Concepts 2
Styles/Mods:
Basalt,
Sandstone,
Garden,
Clock using game elements