The current best custom roms of 2013 so far would be Illusion Rom and Slim Rom and  I have it personally running on my Galaxy S4 and it is solid.  I ran both of these roms on my Phone and they are smooth and solid.  Some love Samsung Thouchwiz but it just doesn’t cut it for me, these roms below give me way better options and freedom.  I can install themes and customize things as I like.  I also decided to flash one of these custom roms because I believe that the default Galaxy S4 Samsung Touchwiz system installs unnecessary apps against my will, like Lookout Security.  Some app makers actually Pay Samsung or T-Mobile to have stuff installed in your rom.  I was getting advertisement notifications before I flashed the Illusion custom rom.  Now I no longer get ads and bullshit.

Liquid Smooth Roms

galaxy s4 liquid smooth  

Slim Roms

http://forum.xda-developers.com/showthread.php?t=2273367 slim rom screenshot htc one galaxy s4