Disclaimer: I go to MIT and am probably biased.

I think the set {MIT, CMU, Harvey Mudd} dominates all the other places you are considering. Roughly speaking, MIT/CMU are the clear best in terms of academics, but if you are worried that the academic programs would be too difficult for you to have time to explore other topics on your own, then Harvey Mudd is a more laid-back school that still has good academics and the sort of quirky student body that might appeal to LWers.

I will also note that MIT is probably unique in terms of the ease with which undergraduates can do research.

In more detail: first, you almost certainly shouldn't go to Harvard. Harvard is an excellent school, particularly in terms of theoretical math and physics, but it doesn't focus much on engineering. You also probably shouldn't go to USC, UMD, or Swarthmore, because the only reason to choose those over MIT/Harvard would be to have an easier degree program, which Harvey Mudd provides while having a better academic program. Note: I am not completely sure of this statement, look up the relative rankings of engineering at USC, UMD, Swarthmore, Harvey Mudd yourself to make sure.

In terms of overall strength of academics as well as student body, {MIT,Harvard} are better than the rest of the colleges you listed, with CMU comparable enough to not rule it out entirely on academic grounds. As I already noted, Harvard is not an engineering school, which rules it out. Therefore, if your goal is to have the best engineering education possible, and to be surrounded by the brightest possible group of peers and instructors, you should go to MIT, or possibly CMU. Further, if undergraduate research is important to you, this is a further reason to consider MIT.

The only reason to not go to the best school possible is because better schools probably have more difficult classes, which might give you less free time. I personally have not run into this issue except by my own volition (I'm currently taking five classes, play sports, and spend 10+ hours/week on research; you only need to take four classes/semester to graduate on time). However, I could imagine this being a problem, especially if you come in without much background and choose a major with many graduation requirements.

The question then becomes how to determine if going to a top university would be an enriching experience or just suck up all your time for 4 years. For this you may want to consider your current background. Are you generally mathematically mature, or will you spend your first semester taking introductory calculus? Do you know at least one programming language? Can you pick up new information quickly? Have you taken at least a couple AP science courses? If you answered yes to all of these, you're probably reasonably well-prepared for any university. If you answered no to all of these, then you probably would not have a good time at the most rigorous universities like MIT/CMU. If it was a general mixture, you could get an improved estimate by looking at typical classes on OCW and seeing if the pace is reasonable or not.

Finally, please don't underestimate the relevance of being able (at a top research university) to interact with the world experts in most fields, as well as extremely bright students. I've discovered numerous academic gems through conversations with these people that I would have been unlikely to get by participating in online communities or reading papers off of google scholar (including entire fields of study / research programs that I didn't know existed).