SuperTab
Perform all your vim insert mode completions with Tab
...Works with omnifunc and external completion sources or frameworks. You can configure direction, cycling, and how Tab behaves at line starts or after whitespace. It is intentionally small and focused, providing a thin layer over Vim’s completion to streamline everyday typing. Lightweight approach that keeps Vim’s native completion model. For many users, it becomes a muscle-memory improvement that speeds up coding with minimal configuration.