That’s a difficult question to answer and one we wish didn’t have to be asked. Different states have different gun laws and some are more strict than others. For instance, some states restrict the capacity of magazines and types of guns you can own based on cosmetic features, while others have no such restrictions. Washington state considers a semi-auto .22 rifle an “assault weapon” and imposes additional background checks and age restrictions. California would not impose the same restrictions for the same gun, however both states have severe restrictions on gun rights.
The strictest states, like Illinois, impose licensing just to own guns and also ban guns based on cosmetic and style features. They also restrict the capacity of magazines. But it’s pretty tough to say “this state is strictest” because each combination of gun laws impacts different people differently.