Best case:
Things get worse gradually enough that we can both learn a more sustainable way of life (most people growing at least part of their own food supply) while being able to maintain a useful level of modern technology in the most necessary areas.
In my opinion that would be firstly, medical technology since you don't want to die of a ruptured appendix or an ectopic pregnancy due to std infection, or in a breech childbirth, or placenta previa, or typhus, cholera, plague, tuberculosis, tetanus, rabies, gangrenous wounds etc. and it would be nice to have anaesthetics, antibiotics and basic dentistry.
The pre tech world was a horribly painful and dangerous time to live in. A badly infected finger could mean the amputation of a hand. Without painkillers, antibiotics or skilled medical attention or blood transfusions, your chances of survival drop pretty quick. I'd be a cripple today if I hadn't had access to a modern hospital a few years ago. I tripped over a frozen chunk of dirt and shattered my ankle into about 6-7 pieces. I needed surgery and a bunch of hardware implanted. We are going to need painkillers at the very least to avoid the worst of a slow horrific lingering death from any number of causes.
Secondly, access to information technology and communications and the maintenance of literacy. The old world was limited in many ways by the slow spread of information and the (literal) perishability of knowledge. If your only doctor or nurse practitioner or midwife dies of something, the group loses all that knowledge and experience. If your only skilled builder and repairer croaks, you'll be living in a mud hut soon enough. If your gardening expert bites it, you starve or subsist at a very low level. Your expert in canning dies, you might die of botulism from an inexpertly canned batch. We need skilled people and can't afford to lose them before they have a half dozen apprentices trained up in the basics.
A lot of human history has been limited by not being able to preserve knowledge in any more secure way than word of mouth. Literacy and books are like a prosthetic memory. A proper interconnected (solar or biogas powered) data and communications system would be even better. You could consult with an expert even if he or she is 1000s of km away (we can beat the line of sight problem with hot air balloons lofting a repeater, or by ham radio). If you have a computer, however DOS basic, you can receive vast amounts of txt based data and hang on to it long enough to transcribe it into hard copy. I've been reading up on hugelkulture lately, and it's got immensely useful applications in drought resistance and growing in poor soil. Last week, I'd never heard of it, but it's a simple and brilliant and centuries old adaptation from eastern Europe that is being used to successfully grow crops on the edge of the Sahara. Readily available knowledge is a lot more efficient than every little community having to reinvent the wheel by trial and error.
Communications over long distances helps maintain the structure of society over an area wider than one can travel on foot or by horse in a week. One of the (many) reasons the Roman Empire fell apart was that it was so big, communications over such a huge area were a serious problem. The famous Roman Roads were their internet. If you can communicate wirelessly, you don't need to travel as much or invest in massive infrastructure projects.
Third and last, I'd like to see more investment in biotechnologies. Growing penicillin on moldy bread is the general idea. If we can make up a basic set of medications produced by genetically programmed bacteria grown on an easily maintained biological substrate (eg. sterilized meat broth), each community can literally grow and maintain it's own medicines from starter cultures. More advanced applications of biotech would be redesigning plants to tolerate a wider range of growing conditions, so your sweet potatoes don't bite it in a freak July snowfall, or your fruit trees wake up too early and catch frost kill.
We could continue to live longer, healthier and more pain free lives than most of our ancestors ever dreamed of under similar circumstances, if we are lucky enough to hang on to a few of the basics.
A good sustainable and safe from bandits transportation technology would be airships. There is a company in Alberta trying to get it off the ground to service isolated northern communities lacking roads and needing high volume cargo. There are rumours that the american military is interested.
(My educational background and career are in Library and Information Science, which probably explains my obsession with information availability.)
