I'm a big fan and proponent of sit, and probably always will be. It's a simple behavior, the first one Elka got, and something a dog can do even on the vet's exam table, if you're waiting. Down works as well, especially if the table they use at your vet's office is also a scale, and thus a bit wobbly and perhaps inclined to make your dog feel unsure about his or her vantage point. Note, however, that I do not have any pictures of Elka sitting, or laying down, on the exam table; I thought it more important to have both hands on her in case she took it in her head to take a leap to the tile floor. That said, Elka will pretty much sit anywhere, though distractions may affect her latency (latency being the time it takes between my giving the cue and her executing it).
"Stand" is a good one to know. You can teach "Stand" with your dog either in sit or down. The way I taught Elka was to have sit or lay down. Then, I took a treat and held it at my side at about hip level. I didn't say "leave it", and so she took immediate interest in it and stood to sniff it (I didn't say stay, either, so breaking position was obviously not a concern). Once she was standing, "Good stand!" (or click) and I gave her the treat. Though sometimes she'll confuse it with "Spin" if she's really excited and wants what I (or somebody else) has, Elka will stand at the word. The hand signal I've used is to show her my right palm, underhand, next to my leg, so as not to be confused with something like "High five"
"Stand" is handy at the vet for blood drawing, and for general examinations. There's a good reason that there is a "Stand for examination" portion of judging in conformation shows; when a dog is standing, you can see all of her angulations, see if she's square, and see if she's sound. Practice this with your dog, having her stand, running your hands on her ribs and down her legs. I will sometimes also say "Stay still" once she's in a Stand (and other positions), something that I really just taught by habituation. Look into her ears, and examine her mouth, doing it positively, and gradually increasing the length of the examination. Teaching "stay still" means, ultimately, that there won't be any jerking away when you have your dog's paw, or ears. If you get your dog used to this at home, the only thing she'll be distracted by at the vet is the environment itself, and not what's happening to her. Actually, now that I think about it, the vet didn't look into her mouth this last trip. I try to pay good attention to her teeth, and let her have plenty to chew on. Plus, we're working on the command "Open", where she'll simply open her mouth when I say open.