Are there any good alternatives to GarageBand available?

Are there any good Linux alternatives to Garageband?

The most important features would be:

• Good audio recording, with audio filters
• Good MIDI editing abilities
• and a good selection of instrument synthesisers, that can be 'played' on a virtual keyboard

Ideally it would be lovely to find all these features in one package but I'm definitely not against using multiple programs.

I know it's a bit too late, but for future users, Ardour seems to be the best option, there's a free and paid version and it is available in Ubuntu's repo, you can install it via

sudo apt install ardour

Audacity (installable in the Software Center) is probably the closest to what you want. It provides most of the functionality you've enumerated, with the exception of MIDI editing.

For MIDI editing I'd suggest Aria Maestosa, though it's not currently packaged for Ubuntu so you'll probably have to download and build the source.