INTRODUCTION AND BUILT-IN METERS
HINT: As of 2006 the best meter for any film camera is a calibrated digital camera. You can use a DSLR's LCD to preview the effects of lighting, light ratios, zone values, color temperatures and everything. It's like looking at a processed chrome on a light table. Once you get the look you want on the LCD you just transfer the exposure from the DSLR to your film camera! Be sure to read here on how to calibrate it to your film camera.
The best meter is the one built into your camera. I always use the matrix meter in my SLR and shoot in Program Automatic. I only mess with handheld meters with my larger format cameras that have no meters built in. I hate using the hand held meters I address below.
If you aren't getting the exposure you want then adjust the camera's exposure compensation, which is usually marked as "+/-" on today's cameras or a dial that goes from +2 to -2 on 1970s - 1990s cameras. Buying a new meter won't fix that.
Built in meters usually read Through-The-Lens (TTL) and consider your filters' and lenses' light transmission which make them more accurate than hand held meters, unless you custom calibrate the hand held meters. Your efforts are probably better spent calibrating the camera's meter. The multi-pattern and matrix meters built into my Nikon and Canon film and digital SLRs and point-and-shoots are far smarter than any professional reflected or incident meter. Not only that, but today they're an integral part of everything the camera does, especially when it comes to White Balance and Flash.
This article is if you are shooting digital or slides. If you're shooting negatives (print film) then you can just guess at exposure unless you're printing yourself. If someone else is printing your negatives then exposure issues are almost always caused by bad printing and not your exposures, although the lab will try to blame it on you.
Ansel Adams used a Pentax Digital Spotmeter. It's my favorite. It was sold new through 2005, and as of 2006 seems to no longer be available new. Before the digital meter, which has red LEDs at the bottom of its viewfinder, Ansel and I used the larger, heavier, more delicate, more precise and less expensive Pentax Spotmeter V which uses an analog needle on a scale. They have the same accuracy, which is pretty much perfect. I have two analog and one digital spot meter and they all agree with each other. That's very unusual. Most meters never agree with each other. The Digital meter used to be sold here and the analog meter used to be sold here. I bought all three of mine used in the 1990s and they've all worked perfectly for years.
You have to use the Zone System with a spot meter.
The Pentax Digital Spot V (analog) takes three S76 or A76 or 357 or LR44 etc. button cells. You can get these at Radio shack and every grocery store.
The Pentax Digital Spot takes an A544 or 4LR44 or L544 battery, 6 volts. You can pay $10 at the camera store, or $2.33 at Home Depot in the garage door opener department. The 544 is actually four of the above cells in a single case.
The orange mark on the shutter speed scale is 1/50 second, which you use in Hollywood for movies shot at 24 FPS with a 180 degree shutter. 180/360 times 1/24 = 1/48, close enough to 1/50.
The weirdest scale is the IRE scale that goes to 100. IRE stands for the Institute of Radio Engineers (!) who defined the scale back at the dawn of television in the 1930s and it is used to this day in video and television to measure the equivalent of the zone system. It is the scale over which you paste your zone system sticker. IREs go from 0 (black) through 50 (gray) to 100 (peak white). Why radio engineers? Simple: that's who invented TV back then. You couldn't have had TV engineers before TV was invented, could you?
There are other bigger, more complex and more expensive meters like the Sekonic, Minolta and Gossens. I find these too big and complex. In the zone system it's easiest to do everything on a simple linear scale, which you just draw or stick on the Pentax meters' dials. The more complex meters lack scales and try to do everything in their own internal computer. Good luck figuring them out; I never have and you probably won't either.
The Zone System is simple when you learn it. A complex meter only ensures you never will. The Pentaxes make it simple.