Acepromazine is most commonly used, though I have heard of some vets prescribing Valium. And often if the kitty is a senior, they'll use diphenhydramine (Benadryl) which is primarily an antihistamine but has sedative side-effects.
I must agree 100% whole-heartedly with Hissy. If your vet ~must~ sedate, then ask him to use one of the gas forms such as isoflurane or sevoflurane and NO Ketamine. The gas might be a tad more expensive, but in the long run it is MUCH safer for our cats.
I too would like to remind about antihistamines as mentioned above. Fex some finnish owners of pedigree sirs use them with good effects.
Ie not for aggressive cats but for male cats to calm down them a little and give them more peace.
Preventive pills is another alternative in these cases - better than preventive injection (these injection has sometimes some nasty bi-effects).