wich is the best on the fly encryption tool that comes with a gui?

Define "on the fly"... look into GPG+gpgme.

on the fly is a tearm used to discribe a special kind of encryption. Basically it means that the files are allways stored in enqrypted format (usually in i special partition) and only decrypted to the ram.
if you want to know more google it.

So, you want an Encrypted File System? The 2.6 kernel series provides EXCELLENT support for encrypted filesystems, at least in loopback.

Look into loop-aes (http://sourceforge.net/projects/loop-aes/). Basically, you setup a partition, but create a separate device to access it. That device consists of the encrypt/decrypt actions for anything passing through it. Then you read/write to/from that device and its all encrypted.

I will also point out something that I realized while reasearching crypto file systems (CFSs). A CFS is of little value if the volume you are crypting is always mounted. If your system is compromised and the volume is already mounted, someone can read/write through the same mechanism that you do, effectively ruining the whole purpose of the CFS. If you have a removable volume (e.g. zip, CD, external HDD, etc), then you need to take care to unmount the volume when it is not in use for maximum security.

Also, encrypted file systems are hard to use for anything that must be mounted at boot time: it requires user input. (I suppose you could store the key somewhere else, but that defeats the point, doesn't it?)