The topic of "best portable" is often discussed on forums; here is a brief presentation of what I consider the top candidate.

Following the end of the Second World War, L. C. Smith & Corona Typewriters, Inc. resumed production of its line of portable typewriters in essentially the same form as it appeared pre-war, with the exception of the temporary exclusion of the small, flat machine.

Above, a 1948 example of the Smith-Corona line; this is the SILENT.  It is equipped with manually set tab stops, and is in the Speedline body originally introduced in 1938.  While these are good machines for use even today, some features are not optimal -- considering the vast range of machines available used at this late date.  Specifically, the carriage arrangements leave a bit to be desired, especially the paper bail which only pulls forward.  The keytops are also not as finger-friendly as those developed for the redesigned model.

This is an illustration for a Christmas ad for the new Smith-Corona portable.  This is the 1950 body; the model pictured is, again, the SILENT -- which was the top model at the time.  It also features rear-mounted manually set tab stops, but these are now concealed and dustproofed behind the new folding rear carriage plate.  The machine also includes the now-familiar (and patented) "rabbit ear" paper supports.  Paper bail and other carriage arrangements vastly improved; keytops are also new one-piece resin with dish for fingertips.  A good deal of internal improvement has taken place, both in terms of (slight) mechanical changes and soundproofing.  This is a good machine to use today; but I prefer the later Silent-Super.

In about 1953, a new machine called the SUPER appeared, which was very quickly renamed as the SILENT-SUPER.  This example shows the first variant of SILENT-SUPER, with 42 character keys and the new toggle-type keyboard tab set/clear control on the right side of the keyboard.

This is the SILENT-SUPER that is much more common; it has two more character keys, and will often have a serial number suffix of "X."  Small decor changes occur over the years of production, but the machine essentially remains the same.  Later on, following the 1959 introduction of the totally new Galaxie body, this older design remained in production carrying various names but almost always being the previous Silent-Super in design.  In point of fact, the Galaxie was 99% the same machine with a complete cosmetic redesign.

One further variant is illustrated by my brother Dave's typewriter above.  This is a TOWER PRESIDENT.  It was manufactured for Sears-Roebuck by Smith-Corona, and is the Silent-Super in every respect.  There were other Tower-branded machines available corresponding to the other models in the Smith-Corona line; the Commander essentially duplicated the Silent. 

These machines are frequently discussed on the Portable Typewriter Forum, and have gained quite a following even in 2005 as among the best-working and easiest to repair machines -- although they're normally found in the field in fully operational condition.  For the modern author, considering the vast supply of these machines still available, and the usual price, one of these should be an early purchase.