If you want realisim, go for high end gas guns, such as the guns from Western Arms ( the king of all Airsoft .45 makers ). These guns are flawless in detail, accurate, powerful, and just a blast to own.
If you are on a budget, go for springer rifles and pistols. The new DPMS A-15 is one of the best - powerful enough to leave a nasty mark, and accurate enough to get headshots ( as in forehead, nose, etc. )at over 100'. It has a mag capacity of 50 rounds, and fires at about 274 FPS - the same as a TM M-16A2 AEG.
For a springer pistol, go for TM HGHU ( high grade hop up ) versions, and the heavy weight KWC models. The best of the TM pistols are the 1911A1 and the MK-23 - both can shoot accurately out to over 120' ( chest shots ), both field strip, and both look very good.
If you want advice on Airsoft guns, come to me. I play huge games all the time. Back in the days of high school, I didn't know much about it, but my shooting buddies resort to Airsoft when shooting at each other... .223s hurt.. : P
Here's four sites to learn about, and buy Airsoft guns:
www.Airsoftplayers.com
www.Airsoftretreat.com
www.shortyusa.com
www.redwolfairsoft.com
No matter what gun you want, it can be done in Airsoft. Any model, any performance, anything.