"Too light" or "flimsy" are - as beak pointed out - two adjectives that I would never use to describe an Olympia SF, which has to be one of the most solid ultra-portables ever produced. It can be argued that the Olympia/Optima Plana is an even more robust machine, but it has a terrible type action, something that Olympia greatly improved upon with the introduction of the SF range of models.

​The Hermes Baby/Rocket has a lighter type action, and is actually a nice machine, but overall it lacks the sculpted-from-billet feel that the Olympia exudes.

​Many would probably cast a vote for the Lettera 22, it is readily available and performs well, but my large-size hands have never easily adapted to the Olivetti keyboard.

​Keeping easy-to-find models on the forefront, I think models such as the Smith-Corona Skyriter, Triumph Tippa, Keller & Knappich Princess, Royal Royalite, Consul 232/311, and any number of Brother models to be worthy of consideration too. There are so many different ultra-portables out there, and thankfully most of them are decent typewriters and only a few would be models that I would avoid at all costs.

steini wrote:

I have one Antares Compact and even is in a really good shape it is not the best typer in the collection, I guess I compare it with the other (bigger) typewriters I use, but it is not my cup of tea.

​Unfortunately no ultra-portable is able to compete with the performance characteristics of a larger machine, definitely not those of a standard; there is a price to be paid for an ultra-portable's compact size and added convenience. I have an Antares Compact and it's not a bad typewriter, there are worse, but the Olympia SF is without question a more robust model, but you should be warned that its type action is heavier.  
 

The 1907 Bennett is known for it's ultra-portability...but it's a pain to use.
I'm not sure what you'd need it for, but my suggestions would be the 1920s Underwood 3-bank.  Very small, very sturdy, and very light.  It has the old cast-iron construction so it's very solid.  The action is a bit loud and snappy, but it's overall a pretty good piece of machinery, though not nearly as nice as larger machines.  The early Remington Portables are very good as well.  Light, low-profile, and very easy to use.  My favorite portable is definitely the 1930s Royal Model P.  It's a bit heavy, around 20 pounds, but it types like a dream.  It has a nice light touch, and a great mechanical feel, and it's very well made, though not the kind of machine you'd want to throw around too much, and not the most "portable".