If you want to be 100% safe - CM7/Gingerbread MIUI
If you're willing to risk your Internal SD card being borked (reason is unknown, although a lot of cases showed up around/after ICS/JB are ported to the 1st gen SGS devices. I highly suspect its a hardware issue, but you never know) - Slim ICS 4.2. It's not as smooth as JB roms, but it's prettier than GB roms, and it's more responsive IMO.
Jellybean option - CM10.1, CyanAOSP, GamersROMz, Slimbean, SuperNexus. Stick to 4.2.2, 4.3 doesn't bring any significant new feature, and the system feels a lot slower than 4.2.2 based roms. CM10.1 is the standard Cyanogenmod option, CyanAOSP is what I used to use before my Internal SD Card borked, it's essentially CM/SuperNexus lite. GamersROMz is pretty fast, but I lost the file and the download link is super slow, so I gave up redownloading it. Slimbean has the cool inverted mode that save battery, but it's a little slow compared to the CM based ROMs. SuperNexus is really fast and smooth, but feature wise, it's like a Nexus. You'll need Xposed to get some features that is built into CM, but at that point it'll slow down.
Don't bother with KitKat IMO, it's slower than JB without any significant functionality upgrade that is relevant on our devices.