The best software? Are you including expensive, paid for (you know, 10k$ per seat and up), cheap paid for for all platforms (the ~500$ stuff) or do you only want the free stuff for Linux? Or some other combination?
And do you want analogue, digital? Or maybe microwave, which is a specialist field all on its own?
And is the need for circuit capture and simulation (or just simulation)? Or is it for a package that integrates capture and simulation with printed circuit board layout?
In the Ubuntu repo, there is Oregano, KSimus Circuit Simulator, and Klogic. Don't know if they're any good, I'm just downloading Oregano to try it out, but they're at least there
Haven't tried oregano much, but the other two are quite limited; fine for what they are aimed at, but it may not be suitable for the OP, that's unclear.
One problem with all EDA software has is that of the learning curve. Even with all of the GUIs and wizards, there is a significant amount of learning that needs to be achieved and it is very easy to get results out of your simulation that have little resemblance to what the real circuit would do and not knowing enough is, in this situation, a little dangerous.
One of the biggest problems that the free options have is that various areas of documentation are often inadequate, or difficult to get to grips with. Either there is no introductory guide, or the reference manual is not as complete or a clear reference as you would like. Or, maybe the direct operation of the software is covered but there is little about the underlying software which is patterned on, eg, spice and you are expected to already understand that from separate sources.
Geda is potentially quite good, but don't underestimate the learning curve. And last time I looked (some time ago) the Ubuntu repos were bad on this; for some reason, the Ubuntu packager had taken one part of the whole package and given it the name of the whole suite, which was a bit perverse and probably confuses a number of newbies to the field.