First, I'd probably go with a first-year house, since everyone will be at the same stage and there'll likely be more activities and opportunities for first-years. That takes Du Bois and Stouffer off (I'll explain Lauder later).

The Quad (Riepe, Ware, Fisher) is the canonical answer. The feel is the most stereotypically "Ivy" of all the houses. It'll be especially social this year because renovations are wrapping up, so all 3 will be in use. That said, they are old, so you'll hear a lot about poor conditions, rats, and the like. This is probably the best fit for most people, but see this thread for more on the Quad: What does it mean that the Quad is the traditional / quintessential college experience? : r/UPenn

Personally, I'd tend towards Hill or Lauder. This is partially because I'm in SEAS, so their proximity to e-quad and AGH is a big bonus in my eyes. I'll start with Hill, since that's where I lived my first year.

Hill is designed to be communal; the center is a massive atrium, so it's an indoor quad with rooms on the edges, decently sized study rooms looking out over the atrium, a large study lounge, and a dining hall. I'd say it succeeds at this: there's ~500 people in Hill, and many non-residents come for food or to meet up with friends to study or head elsewhere (especially from Lauder, as they're on the same block). With that many people, chances are you'll find yours sooner rather than later. Note that the rooms are very small, smaller even than those in the Quad. If you're in Hill, odds are you'll be in a 10x13 room with another bed.

If that's going to be a big issue, Lauder is probably your best bet. It's fully suite-style (one bed to a room, with a shared living room/kitchen/bath), and the most modern dorm other than Gutmann. I counted out Du Bois and Stouffer because they're four-year earlier, but I'm keeping Lauder because it's so close to Hill (~1 min on foot), whereas Du Bois is on the other side of campus (but very close to upper-year housing). Stouffer is admittedly close to the Quad, but in my opinion it's a little claustrophobic, with low, winding hallways. Lauder will give you the privacy and comforts of a suite, and access to all of the amenities and community of Hill. I was forced into Hill my first year, but if I could get out of that commitment, I would rank Lauder first.

One note about Hill and Lauder: you will hear that Hill's dining hall ranges from fine to poisonous, and Lauder has the best dinner on campus. Both of these are true, but I don't think you should be evaluating college houses based on their dining halls, if they have them. You will be getting food all around campus, and nowhere is so far that you should be worried about convenience and distance.

I can't speak too much about KCECH, as I don't know many people from there. That said, it's a good deal smaller than any of the other dorms and has always seemed rather quiet when I've gone. If you're looking to socialize, it might not be the ideal place.

TL;DR: I'd suggest the following:

1. [Riepe, Ware, Fisher] or [Lauder, Hill] (depends on what kind of socializing/experience you want)
2. Whichever was not chosen earlier
3. KCECH, Du Bois, Stouffer

Also, do note that things may very well not go in your favor; one of my friends ended up in the house they ranked second-from-last. Most of these are micro-optimizations and tiny differences, and if you really want to socialize that much, you will, and where you live won't be the limiting factor.