None. Android is very secure on it's own. Lookout comes installed, so if you want a security blanket then that will be fine...though I have it disabled.
The Samsung+ app that comes preloaded also has diagnostic and cleaning tools built in.
Most utility apps really don't increase your level of protection at all, nor do they provide any additional functionality. They do, however, tend to tie up system resources, drain your battery faster, and collect user data.
They're kind of like having central air conditioning in your home and deciding to install window units. As long as you stay away from shady websites or installing apps from unknown devs, you'll be fine.
I basically agree with Brew except that I feel a bit more need to be vigilant. <sarcasm>Junk Cleaners and their ilk are very helpful for people who cannot think of any other way of getting lots of ads </sarcasm> The air conditioner analogy is brilliant. I agree that Lookout is as powerful as you need, but make sure that you run full scans regularly and pay attention to the result. "stay away from shady websites" is good advice but you cannot always tell that the site is shady. If you realize that you loaded a website you shouldn't have or you are not sure, have Lookout run a full scan. Also, do not open any email attachments unless you know who sent it, you were expecting it, and it is not a .zip or .exe file. Ask your email admin what antispam service is checking your incoming messages. If s/he does not know, you may be in big trouble. Do not click a link in an email, type in the URL into your browser, its more work but way safer. If an email sounds too good to be true it probably is, and if it gives you a panicky feeling like, I better click this right away or there will be a disaster, stop and carefully consider the possibility that you are being hacked. It is fine to use free wifi in coffee shops, airports, etc but while you are connected do not type any important passwords, access any financial accounts, or download any files. When in doubt, ask yourself, if I were seriously paranoid what would I do, and then do that. Security wonks would call all of this "just use your common sense" but people who have a trusting and generous nature need to practice this stuff in order to get it right.
For the most part, it's still quite difficult to get an actual virus on your phone, because malware requires you to manually accept the installation (which is why they try to fool you into thinking you're installing something legitimate). Use common sense:
1. Avoid shady websites that deal with things like porn, gambling, and "free" (aka pirated) apps/music/movies.
2. Never ever tap on a link that appears in a popup while browsing, especially if they're warning you that your phone is infected--they're just trying to scare you into installing some bogus "antivirus" app that is probably malicious itself.
3. Only install apps from well-established app sources like Google Play Store or Amazon Appstore. Read a bunch of app reviews before installing an app to look for any complaints about adware or suspected malware.
4. Turn on Google's "Verify Apps" function. This allows Google to periodically scan your phone's apps to look for malware. It's usually in your Google Settings app, under Security (although it might also be in the main System Settings, in Settings>Security).
5. Turn off "Unknown Sources" in Settings>Security. This prevents any app that wasn't obtained from Google Play Store from being installed (which could include malicious apps that are inadvertently downloaded).
Welcome to Android Central! What do you mean by this? Are you suggesting the OP just look through the Play Store for an antivirus app? This isn't a great way to find a good one, since there are most likely a number of poor quality antivirus apps there, and it's difficult to tell for sure just from user reviews.