Since what I need is a cargo bike as well as a commuter, *and* I need a non-saddle seating solution that lets me sit low on the bike, to easily place both feet flat on the ground when stopping, my idea of a Best bike is likely very different from most.
That said, the basic shape of what I need is a lot like my CrazyBike2, but it'd need to:
--have a seat-mount that is adjustable front-to-rear of a few inches, to allow different leg-lengths to the BB. The BB itself could be adjustable instead, but this complicates chain length control.
--handlebars use remote steering to the front wheel, preferably by rod-with-bearings, but cable or chain control could work. Stem for bars would allow for adjustment of bars to arm length and seating position.
--make parts of it modular, rather like the Xtracycle is, so I can use it with or without the side racks. Side racks would be behind the seat but primarily in front of the rear axle, and fully supported by the main frame so that the rear swingarm is totally separate.
--top rear rack would be a bonus, preferably either removable or adjustable so larger diameter wheels can be used than it might otherwise allow for, and/or so the rack can be lowered if the shock is locked out, and lower COG with cargo is required.
--option for a second seat instead of the rear rack would be nice.
--have a trailer hitch (to haul at least another 500lbs of cargo in addition to the 200lbs minimum on the bike).
--have the battery space in the frame *below* the top tube, as my two semi-recumbents do (along the axle line). In addition to helping balance out the bike, this also allows for the option to easily enclose the entire frame area either for appearances, anti-theft/vandalism, or aerodynamics reasons.
--Integrated threaded "rivnuts" in the frame area for battery-securing mounts would be nice.
--thru-frame wiring harness openings, to help protect (and possibly conceal) the wiring looms.
--in addition to clamping dropouts front and rear for powerful hubmotors, it would have to be able to easily mount various non-hub drives to it, in a way that easily allows a jackshaft to combine the motor and pedal outputs into one to run to the rear wheel.
--be a very stiff space-frame, to prevent chain misalignment under high-power loads (such as startups with full cargo loads), for chain drive systems.
--full suspension, adjustable by the rider without special tools (an airpump at most) for various loading and road conditions, with individual lockout for both front and rear. Preferably dual rear shocks, and both front rear being hydraulic/pneumatic.
--Assuming a full bike is an option, it would also have to have full automotive-brightness lighting, including side-markers and turn signals and brake lights, at least as an available option if purchasing a complete bike.
--Adjustable headset angle, so different forks and wheel sizes could be used, without destroying the geometry.
--Just about all the points Liveforphysics asked for, as well.
Probably a lot of other things I'd require of a "best" frame, if I ever had the money to buy one instead of building it myself.
But a bike like I describe above would probably cost several thousand dollars each to produce as a complete bike, given that there would only be a few hundred people that would ask for one (but I bet that many people would want it if they tried it out and it was as good as the idea in my head is
).