My favorite forend grip - Hogue AR-15/M-16 OverMolded M-LOK Hybrid Forend Polymer Grip

These are the ones I always come back to. I love the hogue rubberized texture, the grip shape and diameter fits well in my hands and the multiple hand positions makes this useful when holding the rifle in different positions. This was my third one. Can't think of any downsides unless you have very large hands or are short on mounting space.