well, let me relate my dogs, ones that i've had, and ones my family has had.. some are mixes and you can kinda see where the two dogs meet and compromise:
pembroke welsh corgi - very easy to train but sometimes goes into overdrive and doesn't care what you want or think
Old English Sheepdog - very easy if you can make it clear what you want from the dog. It's like teaching an idiot so you must be extremely consistent and firm, VERY eager to please but can be extremely stubborn if you weren't consistent and firm.
Dalmatian/Border Collie mix - Extremely smart, but extremely selective hearing. Therefore, you get a dog that listens intently as you tell it to sit, then goes off and digs holes in the yard instead of sitting.
Beagle/Cattle Dog mix - extremely smart and willing to please, but if a more interesting smell or fast moving object comes along, it's all over. Impatient and easily bored by repetitive training (more than three times).
Beagle/Am Eskimo mix (we think) - provided you have a treat, you are the world and your word will be obeyed. No treat? Goes off and digs holes in the yard with the other dog.
Cattle Dog/German Shepherd mix - very smart and willingness to learn new things, fiercely loyal.
Now, I've heard great things about English Shepherds, which is my next dog, in terms of trainability.