favorite: the large flat grey moon landscape piece with the mountain and crater on the sneakerpimps album cover.
Someone made a brick-built variant of this for a 48x48 baseplate using parts salvaged from the Starwars set 75334.
These sets were allegedly terrible and were being sold on clearance everywhere. They had some cool minifigs so people were offloading the bricks only on eBay for cents-in-the-dollar.
I now have two nice craterscapes for my Galaxy Explorer 10497.
Obligatory favourite brick; classic castle wall with cobbled window 4444p01.
Obligatory least-favourite brick; classic corner piece 4737. It was held by one stud top and bottoms and made for really unstable models.
My favourite is 39793, the so-called 'biscuit', because it's really versatile and provides so many different connection opportunities. My least favourite is a bit more decisive, but it's probably part 15100, because it's way harder than it has any reason to be, to insert and remove from other pieces.
The Ornithopter set calls for putting that 15100 into two stacked 63782 after they’re held together with a 32062 and it is such a pain. I ended up doing it in a different order, getting the 15100 into one 63782, then the other, THEN putting the 32062 through them. Eventually though I realized the 15100 seats a lot easier if you twist it while pushing it though, and that made my life better.
My favourite piece is the plate 2x3, because it is the best piece to build flowing landscapes.
There is no single "least favourite" for me, but a group: Any single-use part, especially the large ones. Like plane cockpits. Or many StarWars things. I just found a LEGO-car sized red dragons head while washing parts. WTF?
Eh, I'll give windscreens a pass, since there isn't a great way to do them using multiple smaller pieces, but that dragon head sounds inexcusable. C'mon LEGO, this is not f*cking playmobil. Assembly is the whole point!
Maybe it’s duplos, but I’m pretty sure if you asked my nephew and nieces when they were younger, disassembly was definitely the point.
I just found it is the head from Set 2507. Special moulded parts for one set, and then painted from several sides - must cost a fortune in production and development. For that money, they could have made a brick-built on.
With windscreens it is a mixed issue. Some have multiple uses, but some are not as good in that regard.
My least favorite is a brown torso that breaks when you change the arms. I forgot old brown was so brittle, and felt really bad that I broke a nice chest piece. Light green Bionicle bones for the same reason, you can't build with them much before they break.