Depends on where you are. In the US, shooting is very popular but it leaves evidence. Bullet fragments, empty casings, etc. Knives work well, but you have to be close and there is always the chance that the intended victim will take it away from you and use it on you. You have to know what you are doing.
Poison seems good. You can do it over time or get some of that good stuff the russians are using.
Run over them with the car. The AG in south Dakota tried that recently. Claimed that he hit a deer. But it might damage your car. Best to steal one for the occasion. You want an older, big, heavy one. Disable the air bags, they might kill you when they deploy.