Your measure of best is a bit limited. For my money "best" should include:
- accuracy
- reliability
- low cost
- readily obtainable materials
- high printing speed
- large build volume
- easy to build, calibrate and maintain
- robust/transportable without loosing calibration
- compact (relative to print volume)
- easy to enclose (for a heated chamber for ABS or to exclude dust)
- capacity to add multiple extruders, either direct drive or bowden
- capacity to add light weight machining head
- light weight - if you plan to move it around. Heavier if you want to limit the effects of vibration.
- ......
You can't maximise all of the above in one machine. Dual direct drive extruders will be heavy and can't be moved fast. A fast delta machine is not particularly compatible with a direct drive extruder so is less suitable for elastomeric (flexible) filaments (and deltas are excessively tall compared to print height).
I haven't built one yet but my favorite mechanism at the moment for my next large capacity XYZ machine is CoreXY. The bed only goes up and down (slowly) and the X-Y mechanism is fast. With XYZ machines like Prusa i3 having the bed moving fast means that the overall machine speed is limited. Also if you do a large print on your moving bed - the moving mass increases and can reach a limit where you skip steps - absolutely the last thing you want to see near the end of a 3 day print! With CoreXY the frame can easily be enclosed and the build volume is large compared to the overall volume. I would bowden feed a diamond hot end if I needed multi-extruder capability. I'd do a quick release hot end so I could replace it with a direct drive single hot end to be able to do flexible filament or a dremel tool to be able to do light machining or PCB drilling.
My Prusa Mendel i2 inspired Repstrap with welded steel frame: [
youtu.be]
And my Smartrap derived Briefcase 3D printer: [
youtu.be]
