This is going to sound totally lame but I love teaching a `sit'. It's a stepping-stone to everything else you train your dog to do, and most often the first thing you ever teach them.
To me it's the first step towards a wonderful bonding training relationship with your dog, and I never, ever fail to get a `thrill' when I see it in their eyes the second it `clicks' for them - they get it, they know what you want, they know what to do, they are all of a sudden the master of their destiny. I love it.
I always teach a sit the same way. Quiet room, no distractions. Sit there reading a book until the puppy/dog gets bored and sits. Click and treat. Read my book. Puppy/dog sniffs around, gets bored, sits, click and treat. I nearly always do it in a bathroom where there's very little to interest the dog except you, so it usually only takes about 30 seconds for them to get bored and sit. Within minutes they'll get it, they'll start offering sits. Click and treat. Shortly after that you introduce the word, and voila! A sit is born.
It's a positive, wonderful way of training your dog to work out for itself what you want, get it right, and get rewarded for it. It makes them feel amazing and it makes you feel amazing.
And, for the record, I have never spent any more than 10 minutes teaching a reliable sit in this way. No matter what the dog. Other stuff is more complex and you can't be quite as simple in your approach. But you can with a sit and I love it.