Interesting thread because after retiring from my proper job, I spent 15 years running my own bike shop so I know a bit about cycling and cyclists. Some of the posters obviously know their bikes but could I offer a note of caution to anybody who is enthused by their comments and wants to get out and get a bike (a great idea), maybe their first for years.
First off, single speed cycling can be brilliant but it is an aquired taste and rarely for the beginner. The inability to freewheel, lack of gears and need to be able of work at a wide range of cadences can be problematic for even the most experienced cyclist let alone a beginner.
Second, With bikes you GET WHAT YOU PAY FOR. Planet X bikes have been mentioned. They are cheap, not because they are brilliant value, they are cheap because they are cheap. Every corner that can be cut is cut. Some you can see but most cut corners you cannot see. Unless you know your bikes, don't buy one that "only needs assembling". Used to take me anything up to an hour to assemble and set up a bike and I had a full workshop with all the specialist kit and I knew what I was doing. There were some bikes, the cheap ones, that I would not go near because I knew they could not be assembled into a bike that was safe to ride.
My advice is to go to a proper bike shop, not a heavily advertising chain promising discounts. There you should get some good advice on what type of bike would best suit you and get properly sized. Most don't appreciate it but the most important thing about a bike is how well it fits you. No amount of gimmicry will compensate for an uncomfortable bike! Try several shops. You will know when you have found the right one, just like you know when you have found the right medical help for your diabetes.
Could go on for pages about bikes!