It depends on what you want to do with it.
vi you will find in nearly every linux distribution by default. Sure the newer ones sometimes switch to "more comfortable" editors. Nevertheless, if you once sit in front of an old box, e.g. with solaris 8 and have to change anything there you would be happy about knowledge on the "old" vi, cause there won't be any nano,pico... by default.
In case the above mentioned "problem" is not applicable for you and you just want the "comfort" of seeing sth. like a controlbar, then I would use nano.
But for me vim is fine.
And here some helpfull commands for vi
i -> Insert
A -> Insert at end of line
o -> neue Zeile
dd -> cut line
yy -> Copy kopieren
p -> insert einfügen
0 -> Jumpt to line start
$ -> Jump to end of line
:0 -> Jump to the beginning of the File
:$ -> Jump to the end of the file
x -> delete character
dw -> delete word