Difficulty is not a primary concern here; the courses listed may range from easy classes to hard classes. Questions of relevance: Did you leave the class with the sense that you learned something meaningful? Did your semester review sheets glow with high praise? A professor change your outlook? Find a course that you could not imagine someone going four years without taking? Please list and help make Williams a better place for all.
Computer Science 237
If you're ever going to want to build a computer out of transistors and wires, this is the class to take. You work your way up from logic gates and down from assembly until they meet in the Dreaded Microcode Project.
This is the best class I have taken at Williams. There is a lot of reading, but about 90% of it is interesting and I'm glad I read it. Professor Wood is also the man, and having him explain history is sort of like having the golden rule explained to you by Jesus.
Math 211 T
Linear algebra is a great class. Prof. Silva might be perceived as a dry person sometimes; but the material more than makes up for it. If you have any interest in science, this is the class for you. Silva is actually often incorrectly perceived as an incomprehensible person. He's very smart and entertaining, If you want an interesting math class, take 211T
Real Analysis lets you see the heart of numbers. You thought you knew what the numbers were? You will see much, much more after this course. If you are good at math and you like it because you are good at it, this class will test you to go above liking math because it is easy, to liking math because it is beautiful and deep. Additionally, Morgan's book teaches you everything you need to know by making you do it yourself, without anything irrelevant.
Math 316/Physics 316
This is the Cryptography course, using applications of Abstract Algebra and Quantum Mechanics. Gotta know a little linear algebra to get through this since the work is a bit tough. If you love good professors, Professors Loepp and Wootters double your pleasure. The first problem set was decoding a long message that was coded with a Vigenere cipher. Good times, tough exams. Definitely do it if you're of the Div3 ilk.
Arguing About God: all the long-drawn out religious arguments we're not allowed to have in polite society. Very engagingly taught by Steven Gerrard, it covers most of the major viewpoints on God and faith. If you've got an itch to debate logically about questions of faith and reason, this is the class to take.
Virtue Ethics tutorial taught by Bridget Clarke. Examines and questions the fundamental assumptions of our everyday unthinking moral thought; suggests an alternative way to frame and speak about moral issues. Makes you reflect deeply upon your own decisions and purpose in life. Cuts through the bullshit and asks hard questions about your own treasured ideas, in a way that few other Williams classes can manage. The readings and discussions in this class may have the effect of changing you.
Social psychology will affect the way you think about things that people do in your everyday life. Seemingly meaningless events, like what gestures you make while talking to someone, or how you feel about something you bought, take on meaning and importance after taking this class. The lecture format that sometimes arises from this course's popularity is unfortunate, but the material is awesome.
History of the Modern Middle East: Blows a lot of your preconceived notions about the middle east out of the water. Adds levels of complexity to issues you thought you understood, but probably didn't. A lot of reading. Most people end this course by falling madly in love with Magnus Bernhardsson who, despite being completely oblivious of it, is extremely funny.
The Apocalypse: Now and Then: Continue your schoolgirl crush on Magnus. And the material is awesome.
Art History 266
20th century Russian art and the birth of abstraction. Definitely the best class I've taken at Williams so far. Taught by Darra Goldstein, who's a reasonable grader, very effective lecturer, and truly delightful person. She does a great job contextualizing the art in Russian history, language, and culture; one comes out of this course feeling like they've learned something about Russian life rather than just the art. No prerequisites, a great class for non-majors, seniors, and senior non-majors.
This course with Garrity is unbelievable fun. If you could not imagine multivariable calculus to actually be interesting, take this class. If you love albatrosses, gnomes and other funny things such as squirrely ds (the letter delta), take this class. If you love a professor who will scream out at random times, wave to passing groups of strangers and ask you to join in the waving, take this class. I know that you fall into one of those categories, thus, take this class.