The simplest and "robust like a Mercedes" gearing solution for urban poke-around is a 3-gear hub. As simple as a single-speed bike, but with much faster starts at the green light! Those hubs have a development of around 186%, which is sufficient for many purposes. (As example, I thook my 3-geared, 16 inch wheeled Brompton on a tour to the Lyons mountains.) If such a thing fits your purpose, don't read any further! Unfortunately those hubs are not very available in North America, Dahon being one of the few bike makers you use them. Check out their "Curve 3D" or the "Vitesse 3D" (the latter one, not marketed in North America, unfortunately). As for Europe, no need to mention any particular brands and models, since hub-gears are widely available (even Dahon has more models using them).
For people who are riding longer distances or hilly areas, a solution with 7 to 9 gears and a development of around 300% will be absolutely sufficient. This can be a hub (three brands offer several models) or a (rear) dérailleur transmission. In both cases you get a linear sequence of gears shifted with a single level and the fastest one is about three times faster than the slowest ones. This covers uphill as well as tailwind situations.
In my opinion, double dérailleurs (front and rear), double hubs (hub gears plus Schlumpf-drive), dual drive (a gear hub plus a cassette) and high-end hubs (Rohloff 12 speed) are a thing for cycling freaks and dorks. Especially the double dérailleur is usually underused (small gears not used by most people), because people just use their mountain bike in the city or on easy to medium trails. Also, the double dérailleur is more complex to shift, and has a lot more wear and tear.
Here's some calculations showing what you really get from a bike that claims to be "21 gears" (or more). Lets call the three chainwheels A, B, C (A being the most-teeth/fastest) and the seven sprockets 1 to 7 (1 being the least-teeth/fastest). Then, the combinations A6, A7 and C1, C2 can't be used because the chain would run diagonally and have too much slack (C1/C2) or too much tension (A6/A7). Besides that, those four transmission ratios are reduplicated by other combinations (which ones depends on the exact tooth counts) and therefore useless.
This leaves us with 17 usable gears, but in reality most people would only use 9 of them and here's why. The fastest gear is A1, from there one shifts down to A2, then A3, then B3, then B4, then B5, then C5, then C6, and finally the slowest gear C7. In short, the number of gears that most people use is only 7+3-1 instead of 7*3. (The same holds for 9+3-1=11 on nominally "27-gear" bikes.) The reason is that most people only use one lever for shifting a gear, and never two, as would be necessary to shift from A3 to B2, for example. What's more, most people wouldn't even know which of A3 or B2 is the faster gear!
The double hub and dual drive (though they have other advantages) have the same problem of messed up gear order.
My advice: even if a bike with double dérailleur looks attractive, don't buy it! A 300% transmission with either a gear-hub or single dérailleur will be the better choice. More convenient, simpler, and more durable. As a very practical extra, those solutions allow the use of a chain guard (the most sporty-looking version just on the chain wheel, as on Dahon's "Speed"). The hub gear even allows a fully covered chain which reduces your maintenance-interval from every couple weeks to every couple years. (Frankly, I wanted to clean Speedy's chain after one year, including rides to slashy snow, but their just was no dirt on the chain!) I think that Dahon's "Vitesse 7HG" (that one available in North America) will fit such a fully-covering chain-guard. It's a matter of ten minutes of mounting this chain guard to saving hours of maintenance later!
Those 7 to 9 gear hubs and derailleurs are widely available in North America (and in Europe anyway) and so I don't need to mention any particular model. As a final remark: the combination Dahon Speed (or any P-range trekking model) plus hub-gears is not available anywhere, so you'd have to build your own, if you wanted to imitate my Speedy. But anyway you'd have to do that because of Dahon's half-hearted dynamo-lights. (See upcoming post.)