What iOS version do I download on my iPhone 3GS?

I have an iPhone 3GS and I have iOS version 4.1. I'm trying to figure out what version to update it to I read somewhere that the highest you could go is 4.2.1. But when I'm trying to update it on iTunes it's saying that the latest version for my phone is 6.1.3. I'm afraid to update it because last time I did it froze on the home screen and I had to take it to a professional to get it fixed. What do I update it to?

For me you should update to ios 6.1.3
I have that on my iphone 3gs and it is working very well

6.1.3 is the latest Apple ios the 3GS will be allowed, take advantage of it! No more support.

An iPhone 3G can be updated to iOS 4.2.1 but an iPhone 3GS can be updated much higher (6.1.3). Do it but backup your iPhone and backup your iTunes library while you are at it. If you don't do it you are going to find it very hard to get any new apps. It is the wealth of apps that helps makes the iOS devices great.