A digital one is nice when measuring low voltages, like the O2 sensor. Otherwise a cheap analog one is handy. I like the way you can see the needle move as a voltage changes instead of watching the digits constantly change.

Skip
'93 850GLT
'83 242TI Flathood

...consider one that can measure at least 10 Amps, for testing Fuel Pump condition/current draw and diagnosing battery drain problems.
--
Bruce Young,
'93 940-NA (current)
'80 GLE V8 (Sold 5/03)
'83 Turbo 245
'76 244 (lasted only 255,000 miles)
73 142 (98K)
'71 144 (track modified--crusher bound)
New 144 from '67 to '78
Used '62 122 from '63 to '67

It depends on what you want to do with it. You need a high impedance meter for doing computer oxygen sensor stuff. And if you have a k-jet Volvo it is handy to get one that has dwell, and tachometer functions built in. Otherwise you need to get a second meter to measure the duty cycle, and a tachometer as well. The one I got was about 60 bucks (Canadian), but that was cheaper than getting 3 cheap units to do the same thing.

Just make sure that the meter covers all the ranges and values of what you want to measure.

I've got a drawer-full of the things, digital and analog, and the one I use 90% of the time for automotive trouble shooting or just checking that everything is OK, is the cheapest little analog rig that Radio Shack sells. It was $14.99 (specials/discounts do happen occasionally). It's no good for ECU or oxygen sensor circuits, but fine for normal 12V stuff, seems pretty robust, and about the size of a pack of cigs. Its main limitation is a max of 250 milliamps on the current scale - about the draw of a 3W bulb.
--
Bob (81-244GL B21F, 83-244DL B23F, 94-944 B230FD plus grocery-getter Dodge minivan, MGB, and numerous old motorcycles)

I would buy one that is easy to use. I picked one up from Home depot and it's like trying to use a computer for the first time, I say just go for simplicity and easy to read. Get one with a speed knob for a selector, not the push buttons