I agree with Zany. I would base it on a couple of variables. What size is the tank? And, how heavy is the bioload going to be?
If the tank is a fish only, less than 40 gallons, and only a few small fish, I'd say you can easily get away with an AC hob filter. In this situation I would make sure to use live rock too. Helps an awful lot with the biological filtration.
If the tank is even a small tank (like the 40g above), but you're stocking it with 8 or 9 (for example) medium fish, I would certainly use a skimmer.
If the tank is bigger than a 50g or so, I would probably use a skimmer regardless of the number of fish.
The above is only a guideline. There is no hard and fast rule about how to go about running a fish only or even a reef. I know people with incredible husbandry skills and run large reefs without a skimmer. This is a very advanced aquarist and I do NOT recommend this to just anybody. I'm only mentioning it to illustrate that there are many ways to do things successfully in this hobby. Certain techniques can outweigh and 'counterbalance' other shortcomings. For instance, daily 10% water changes can make a skimmerless 210g medium stocked fish only tank a success. Do I recommend this approach? No.
Russ