Martin is right: Under any and all circumstance the friction between the tyre and the road determines and limits braking power.
Furthermore, I'd be most surprised to discover even one rear disc brake/front whatever other brake that is more effective than rim brakes all round; the rear brake takes care of perhaps 20% of braking, and that would be under the most favourable circumstances; normally it would contribute less, usually only keeping the bike straight and the rear wheel from overtaking the front. It seems to me a disc only on the rear wheel, unless it solves some other problem, typically on converted bikes, is likely to be wasted.
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A couple of other braking options that are sometimes overlooked:
a) Shimano's roller brakes are a killer option. I don't know what model number they're up to now, but I have the -70 series roller brakes on my Trek Smover bike and they're as good as discs, except better in that nothing can splash on them because they're fully enclosed. Personally, I hate disc brakes and roller brakes both because they demand too much of my attention under threat of a faceplant through too-sudden braking, but there is no doubt in my mind that both offer tip-top braking. The Shimano roller brakes are compatible with Shimano hub dynamos and their reaction arm attaches to standard disc brake fork-side fittings. A novel use of earlier, weaker Shimano roller brakes is to fit the modern hugely effective roller brake on the front wheel, and an earlier series (I used the -41) on the rear for a very agreeable form of anti-skid braking, though you cannot do this on a Rohloff-equipped bike as there are no roller brakes that will fit a Rohloff's four disc bolts. Roller brakes require a service of a special grease squeezed in through a nipple on the outside which can be usefully combined with the Rohloff oil change and EXT click box service.
b) Having said that, on my current daily bike, which I've kept longer than any other bike, I have the biggest hydraulic disc brakes you can fit, front and rear. They're a full 622mm, over two feet across, near enough rim size. They're misnomered Magura Hydraulic Rim Brakes, they're sealed for life, they're never serviced though rebuilding parts and fluids are readily available if you can be bothered (the rebuilding parts and fluids add up to around half the street price of simply fitting brand-new when the old ones suffer some incident; like a Rohloff, they don't seem to wear out). The great thing about the Magura Hydraulic Rim Brakes is that the ones that fit touring bikes once upon a time came in two versions with different size chambers. I deliberately specified the weaker version for more progressive braking; I've never once wished for stronger brakes. (When the usual testorone-for-brains jerks on a bike tech conference I belong to sneered and screeched about how real men need the biggest, harshest brakes imaginable, a famous bike mechanic remarked that actually mine were the biggest disc brakes: full rim size.) As a small bonus, the Magura rim brakes are very economical on brake blocks, and your rims too. If your bike has the fittings for the Magura Hydraulic Rim brakes (if you"re lucky, it's a straight swap for the non-hydraulic fitted brake), I highly recommend them for touring bikes as a perfect, service and glitch free component. BTW, you can buy the cheaper version (HS11) with confidence. Technically it is the same as the more expensive version, just presented differently (and I think more attractively), as Magura no longer offers two chamber sizes in that line. The last time I looked there were several other types of the Magura Hydraulic Rim Brakes for different niches, including some with greater clamping force; if you consider these, you need to pay careful attention to the tyre width it will fit over. If you want to adjust the clamping strength of the touring type it is easily done by fitting or removing the so-called "brake booster", which is an upside-down U-shaped device for reacting brake torque before it gets to the fork; I specified my bike without; if your daily commute is over the Alps, by all means fit it, but for normal riding it is dead weight (if men can self-identify as women, I can self-identify as a weight weenie!). Magura's rim hydraulics are sold singly or in pairs at a saving over buying two singles; front and rear brakes differ only in the length of prefilled tubing and each is fully-selfcontained.