I have tried all of the following AR grips:
stock plastic A2
Ergo original hard plastic (right handed)
Ergo original rubber overmolded (right handed)
Ergo "Deluxe Tactical" rubber overmolded (ambi)
Hogue
Magpul MOE
Command Arms Accessories modular plastic/rubber grip
Advanced Technology (ATI) hard plastic grip with finger grooves
Among all these, I like the Ergo "Deluxe Tactical" rubber overmolded best for shooting at a bench or other short-duration shooting. Extremely comfortable for short periods, but becomes a lot less comfortable after 5+ minutes of continuous holding the grip. It's also the largest and most expensive grip on the list above.
For longer-term shooting, it's about a three-way tie among the Ergo original (rubber overmolded), Hogue, and Magpul MOE. Here's some comments on each:
Ergo - comfortable, fits nearly any hand, not very hand-filling
Hogue - quality construction, nice feel, but does not increase the length of pull and that's a downside compared to others if you have large or XL hands
Magpul MOE - well made and fits most hands, does increase length of pull, but is hard plastic and not all that ergonomic or comfortable as the rubber overmolded grips
The Command Arms Accessories modular plastic/rubber grip was such a disaster I returned it. The inserts fit too loosely and the attachment point was molded too small so I couldn't install it.
The ATI grip is pretty good for the money, but the finger grooves are too deep for my taste and for about the same price I'd rather have the Magpul MOE. Also, its attachment point was molded a bit too small/tight and I had to dremel it with a rotary file to get the grip installed.
All of these except the CAA seemed well made enough to use on a "duty" gun.
The Magpul MOE and Ergo deluxe tactical use special mounting hardware (included with each). You can't use standard hardware with either one due to their size differences. I don't like Magpul's bolt - it has a big screwdriver slot, which is difficult to use compared to an allen head, and it comes with loctite on the threads so you're using loctite whether you like it or not. The Ergo's bolt is simply a longer allen head bolt and I have no complaints with it (it also uses a lock washer instead of loctite).