Best headlamp bulbs to get for a W124?
 

I know a lot of people like the Euro headlamps. I saw some Xenon's on YouTube, but I guess those would only be if I converted to Euro's.

But short of getting those and sticking with the original headlamps, what opinions are there for the best bulbs out there? Does either the 9004 or 9007's work?

1991 300d, 185k

The 9007 swap is relatively easy, just swap a couple of wires. Definitely a worthwhile upgrade.

If the headlamps were designed to take a 9004, that is the only suitable lamp. Likewise, if they were designed to take a 9007, that is the only suitable lamp. The design of the two lamps is completely different, and using the wrong one will ruin the light distribution of the headlamp. A quick Google search seems to indicate that your car takes a 9004, if that is the case your best option (aside from replacing the headlamps with higher performing units) is to pick up a set of Philips Xtreme Vision or GE Nighthawk Platinum lamps, and make sure you have as little voltage drop as is possible to the lamps. The relationship between voltage and light output is not linear, and just being a couple of volts down can cost you more than 30% of your light.

No it won't actually, it works very well. The 9007 has an axial filament vs the 9004 transverse filament. While the 9004 puts light all over the place including upward while the 9007 focuses the light onto the road. 9007 bulb in a 9004 W124 housing is huge improvement both in beam pattern and light output. It should have had these bulbs from the factory. Also works just as well in the W201 housing.