What's best really depends on the user and on the situation it will be used in.
Really, nothing is necessary. A foregrip is nice to have, as it's comfortable to hold the weapon with a grip. But it's not necessary. Iron sights are all that's absolutely required for outdoor daytime games. A red dot or 4x or less optic will be useful for outdoor games.
A red dot will be useful for indoor cqb games. If you play night games or indoors, a flashlight may be useful. Lasers can be useful, but some places won't allow them due to potential eye damage.
And if you like the tacticool look, then whatever you want to attach is up to you. Just remember that you're the one that has to carry that weight around with you when gaming the gun.
Personally, I tend to take the minimalist approach. My C8 has a flip-up backup sight, and an EOTech 552 replica. My M15A4 rifle has an 3.4x Elcan M145 replica. My Type 56 and MP5 are bare. My only gun with full rails, my HK416, has a vertical foregrip and an 3.4x EOTech 552 replica. The rest of the rails are covered. It's no über-tacticool gun, but looks good and will serve its purpose well. The only other accessory I could want to add would be a flashlight if I was playing in low-light conditions. Otherwise, this minimalist approach works best for me.
