Well, right now, if you're building your own rig, there are two realistic options.
The first is the economy option. The obvious choice is the AS-1200. It's a shutoff valve, and would cost you all of $12-15 shipped, new, from SMC, the manufacturer. It has its drawbacks - it's tiny (and therefore fragile), the knob is tiny (and difficult to turn in tiny increments), and in order to put anything on top of it (directly connected), you'd probably have to use both a stainless 10-32 nipple and an epoxy sealant (such as Loctite). And, you'd need a couple 10-32 to 1/8 adapters, which generally aren't available at brick-and-mortar stores. Alternatives are the Pneumadyne and Clippard manufactured valves, which are similarly priced and similarly effective. And both have optional larger handles.
The second option, which costs $10-15 more, is the Parker HR-3 valve currently available on Evilbay. It's currently the most impressive value of all the metering valves anybody has *EVER* realistically suggested for this hobby. I say "realistically suggested" because the super-precise valves are rarely, if ever, available for non-retail-prices. Not the absolute most precise valve, but close, at a fraction of the cost you'll likely ever find for a similarly-effective valve. More precise valves exist, but they're overkill, in my, and anybody else's, *honest* opinion. The HR-2, HR-1, and HR-0 valves would theoretically be more precise (theoretically because nobody here has posted their own comparison test results), as is the Hoke 1600 series valve (which I *have* tested against the HR3 and many of the other low-flow valves). In real-world applications, the HR3 blows away 90+% of the valves on the original list.
Anyway, *right now*, the HR3 is the unarguably best choice for an economical high-precision valve. In a month, or several months (whenever the current Evilbay seller runs out of stock) there will be a number of other high-precision options, especially the Swagelok S series, but also the Swagelok M, Swagelok 21/22, Ideal 1 series, and a few others. They'll cost more, and/or be harder to find. And they'll likely need adapters from 1/8 or 1/4 tubing, or 1/4 M/FNPT.
The one other thing that's really important to remember is that a valve with a large handle is far superior to a valve with a tny handle. For example, a Swagelok S-series valve, with a standard tiny handle, is functional. That same valve, however, with a vernier handle (whose most effective characteristic is not the gradients on the handle, but simply the increased diameter of the handle) is extremely precise. Why? A vernier handle circumference is 2.6 inches, while a standard handle circumference is 1.5 inches. So, a 1/2-inch turn on a vernier handle is a much smaller change than a half-inch turn on a standard handle. For this same reason, you'll see people recommending using a pair of pliers to adjust the standard needle valve on of of the generic regulator rigs (Milwaukee, Aquatek, Azoo, and clones) - it increases the functional diameter. A larger-diameter handle equals more precision, regardless of the valve. And the HR3 has a large handle, on top of its impressive precision.
That said, I'd love to get my hands on one of the Brooks valves, if only for the sake of experimentation. Most, or all, of the variants look like they'd be perfect for us ...and that digital knob looks pretty freakin' cool