Answer: Of all the animals you mentioned, in my experience the most destructive would be the groundhog. They will chomp plants to the ground repeatedly and are voracious eaters. Rabbits and voles will damage woody plants at the base and voles will devour plant roots and lily bulbs. The squirrels will dig and eat bulbs, chipmunks may also eat small bulbs such as crocus and occasionally munch on a flower or two. Slugs do not usually cause excessive damage in a garden unless the population is high, but they will leave a trail of slime so if they are the culprit you will be able to track them.
You might consider having a professional trap and remove the groundhog, be very careful about cleaning up spilled bird seed so as not to attract rodents, and possibly control the slugs by sprinkling diatomaceous earth on the soil surface around the plants. You can also trap them using commercial slug traps or saucers of beer. If bunnies are a problem you could use a commercial repellent or a home made one based on garlic and/or red pepper. Commercial repellents may also help deter other animals, but a fence is really the most reliable control.
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