For security - first of all, try to not install anything from other than Google Play sources. Just because you can - doesn't mean you should
Aside from that there are plenty of antimalware programs which has almost no effect on performance or battery life. I prefer the "Lookout" app - it is free, completely automated and has good detection rates.
For music/video player - MX Player plays just about everything.
For "responsiveness" - it is lightning fast in its stock form, even with " Power Saver" option (which throttles down the CPU) enabled.
For other apps - just browse the Google Play store, you'll find plenty. All depends on your needs. Just wanna mention a couple of them: as a flashlight app I prefer TeslaLED - it is free and has no built-in Ads, and for Apple's "Find Friends" replacement there's an excellent app called "Life360", which does the same and is cross-platform and free. Oh, and for Apple's "find my iPhone" service - Android devices now have same service directly from Google themselves:http://officialandroid.blogspot.sg/2013 ... droid.html
Also, before you finally switch off your iPhone - don't forget to disable "iMessage" setting on it, otherwise you won't be receiving any SMS messages on your new Android phone from your iPhone-using friends
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