As previously stated, "Best" is somewhat of a broad term. "Best," in terms of airsoft, is basically what works best for you, your gear, and your style of play. First you have to gauge what you are looking for in a "vest" - functionality, "fashion" (going for a certain look), or both.
You basically can break it down into three catagories: Chest Rigs, Vests (commonly zip up the middle), and Plate Carriers. Chest Rigs and Vests commonly come in two styles, MOLLE (for customization) and preset pouches. Plate Carriers normally are set up for MOLLE.
In General, chest rigs and vests are lighter and more flexible - good for "Hi-Speed / Low Drag." The ones with pouches sewn in are usually more economical, though you sacrifice the customization offered by various specialized MOLLE pouches - which purchasing "a-la-carte" can get pricey.
Plate carriers are generally heavier and more robust (though there are some "lean" minimalist carriers in the market) specifically for their described purpose. They usually offer larger "real estate" for MOLLE pouches. Some offer a quick-release system (i.e. CIRAS) for emergency doffing (i.e. drowning, medical accessibility, damage, etc.), which is functional in a real battlespace. I personally don't care for the quick-release carriers as they rarely serve any functional purpose in an airsoft battle.
I personally have an assortment of vests and plate carriers for both function and "geardo" purposes. Sometimes I don't even use one, opting for a "battle belt" kit. Just depends on how I feel for that skirmish and/or the demands of the mil-sim operation.
Though, if I were to pick one out of my collection, it would be my Blackwater Gear I/O plate carrier in MultiCam. They are somewhat of a collectors item and, at the time I purchased it, it was hard to find any plate carrier in MultiCam. It's also set up "heavy," with a hydro carrier, gadget pouch, med pouch, com pouch, and space for a dozen magazines in double stack open tops -that's capacity for over 2400 shots in Hi-Cap, or 1440 in Mid-Cap