I would not remap any of these:
<C-@> and <C-A>: Repeat last insert (and automatically escape with <C-@>). These are basically the same as <C-R>.. Indispensable once you get used to them. <C-D> and <C-T>: Change indent level even if you're not at the beginning of the line. Handy, but I don't use it much. <C-E> and <C-Y>: Copy character from below/above the cursor. Sort of useful, not terribly. But not having them would drive me nuts. <C-G>: A prefix key. I don't really care about <C-G>u to split the insert into multiple undo levels. Almost no one uses <C-G>j/<C-G>k to start a new insert a line down/up at the same column the current insert started on, but I'm a huge fan. I wouldn't touch this. <C-I>: This is literally the same as <Tab>. Don't you dare touch this. <C-M>: This is literally the same character you get from hitting Enter. Don't you dare touch this. <C-N> and <C-P>: Basic word completion. Extremely useful. I would never touch this.