I'll respond since I started it. I really do believe that serious 'ukulele-specific instrumental study (note: not ALL musical study on the 'ukulele) should begin with a soprano. My reasons are as follows:
1. The soprano's diminutive size is inextricably linked to the 'ukulele's traditional identity as an auxiliary chordophone for the soloist.
All 'ukulele players will at least occasionally need a soprano for THAT sound -- brash and piercing, the sound of the soprano cuts through most other instruments in a string ensemble. This makes it exceptionally well-suited to melodic and/or dense rhythmic assignment within an ensemble. The larger sizes are also capable in this role, but a good soprano and a matching technique make quick work of the sonic demands in acoustic settings.
2. The soprano 'ukulele's envelope of note production encourages the development instrument-appropriate right-hand technique.
The abrupt attack, rapid decay, and diminished sustain of the soprano places onus upon the student to develop a sophisticated, technical right hand approach to rhythm. This leads in turn to the achievement of lively rhythmic accompaniments. The larger sizes, with their slower decay and longer sustain, invite students to postpone or neglect the development of right hand technique beyond basic strokes and finger picking.
3. The low string tension and short scale length ask the student to listen in order to correct intonation.
The soprano, being susceptible to issues of intonation, asks that the performer make the necessary accommodations in order to produce the correct pitch. This includes modulating pressure applied through the fretting hand, bending the string to make subtle alterations, modifying string displacement prior to release applied with the plucking hand, etc.
4. The short scale length entices students to access the upper frets.
The close spacing of the frets invites study of the upper reaches of the fret board. Happy discoveries through experimentation with regards to note selection are frequent, and moving between positions requires little effort.
5. The soprano is intimate and inviting.
I trust I don't need to explain this one.

Musical achievement is sometimes elusive; we all need our early encouragements.