Which ios is best for iphone 4, 8gb with a 2.1gb of memory free space?

I have an iphone 4 8gb. Which ios is the best for my phone? Now i'm using ios 6. Sometimes it gets lagged when opening app such as line, path or instagram.

Does the phone memory affect the performance too? Currently there's 2, 1 gb of free space in my phone. I don't put any musics in it. I only use my phone for communication and social media and gallery as well.

Anyone know well about ios and iphone, your help is very appreciative.

I would wait to upgrade to iOS 7, wait until you upgrade to a new phone. IOS 7 is really good but older hardware won't have the advantages of the new. (When you do upgrade you can trade it in also and get $ off)