Check out the the HK416, as mentioned. I'm fond of the SCAR and M4 myself, and the HK416 looks like a nice cross between the two
Some good M4/M16 models would be Classic Army, Tokyo Marui, King Arms or Systema PTW (if you can afford it, its probably the best one)
From what I know VFC makes some really good SCAR rifles. They have the Light, Heavy and SSR (Sniper) variants.
For G36s, Classic Army and Ares/Umarex make the whole family of G36 rifles. KWA makes a good G36c as well.
All the ones listed above are AEGs. For gas guns, WE or KJW M4, WE SCAR (latest version only, older ones have some problems) and WE G36 series.