I like Geany for text editing, but it struggles with very large files.
vim is a command line editor that can easily handle large files but its not at all user friendly - the keyboard commands can be quite complex.
Fortunately there is a gui version of vim called Gvim. It has menus, so you don't need to remember all of the vim commands and it handles very large files. Really handy if you want to do a search and replace with a 180mb sql text file.
Install gvim from the Ubuntu Software Centre