I would like to suggest the Olympia Splendid.  Small enough to be truly portable and solid enough to be reliable.  Ready-available ribbons are a real bonus.  The Olympia Traveller is a good second choice, but if you can get one, the Splendid has the edge on quality.  If you want a lighter machine, the Adler Tippa is pretty good too.

Yes, I wouldn't say that the Brother is bad either !  With the exception of the very last models - which were definitely built down to a price - Brother was always a cut above the other main Japanese makers, Silver-Reed and Nakajima.  I am sure that in the mid-1960's they were trying to emulate Olympia.  They never quite got there, but it was a nice try and as a result the machines were good quality.  You also have the advantage of a readily-available ribbon, as you do with Olympia.

A mid to late 1950s Smith-Corona Skyriter or the Sears version, Tower Chieftain. Either one with the longer carriage return lever.
My Tower Chieftain III is late 1950s and it has a very snappy typing action. Absolute joy to use. In my view, better than the Olympia Splendids (33,66,99) and SFs of the 1960s.
Groma Kolibris are nice, but pricey, and they can be quite loud if the platen rubber has hardened.
Olivetti Lettera 32 is nice, but to me, it has a sluggish feel to the keys. I've had mine since new, when I bought it in 1981. Sentimental value to me, but I rarely use it these days.

 

I'm a bit late to the party, but I'll vote for the Smith-Corona Skyriter.
The earlier ones (two-tone green keys, tiny return lever, metal shell case) seem to be made better than he newer ones. Even the all-white key versions feels mushier to me, but that's a persnal preference that you will have to decide for yourself.
I will admit that the Kolibri feels like a swiss watch compared to the sloppiness of the Skyriter, but the Skyriter was made for mass production, and they are very reliable.
The platen is -just- wide enough to insert a #10 envelope to type an address, unlike the Groma Kolibri.
There are 10 chars per inch (Pica) and 12 chars per inch (Elite) models, and my personal preference leans toward the Elite typeface; it seems cleaner.
THE BAD: Skyriters do not have a Tab key, and they don't have a ribbon color selector - Black is all you get.  They use a slightly rare spool (1-5/8", or 1-9/16"), but they are available, and once you get them, you can certainly wind your own inexpensive ribbon onto the spools.
THE GOOD: The keybard action is great - with a heavy -- light touch selector, you can choose your keyboard "feel." Smith-Corona made a jillion of them so you ought to be able to find one you like for less than $100.00.  $15.00 is not unheard of in thrift stores or garage sales.
Size-wise, the Kolibri is about the same footprint, and 3/4" flatter than the Skyriter, but in he travel cases, the metal-cased Skyriter is considerably smaller. If ou want to take it places, you can put about a dozen sheets of paper in thhe case, and if you are good, a pair of ribbon spools in the basket. 
-- Ardie

I can also vote for the Skyriter. Just got one last weekend for $10. This is the version made in Great Britain, which supposedly aren't as nice as the earlier ones, though I'll have to test that out. I like it very much, actually -- nice feel to it, and the print (pica) is very well aligned, something I was thinking might be a problem. While a ribbon color selector would have been nice, I can live with it. I got a shorter spool of cash register tape (purple color) and spooled it on; works fine. The line spacing lever is a nice short throw, like all SM portables from the 50s forward. I think this machine would be good for lengthy typing stints, something I don't think I can say for the Lettera 22.