I started playing before you could buy a compressor in a pedal left alone an optical model. However, for many guitar and bass players think it is the most useful tool you can have. To me, the Robert Keely Compressor, which seems to be an update of Ross, is the only one worth buying, but you might also want to check out Demeter or Visual Sound. The Joe Meek model looks very interesting. Some old units squeeze the sound so much that it colors the sound which can be fun.
Old school electric bass players might want to check out enhancers such as the BBE, or Aphex. I thought the Aphex was hard on batteries and less modern sounding.
They can be temperamental, but part of the fun, a geranium 60's Fuzz Face is helpful. Silicon Fuzz Faces are more stable, but brighter and heard on many classic rock albums. Many like the old green Maxton/Ibanez 808/7 overdrives. Keely makes a version of both of these if you use them a lot. A Bixonic Expardora should have a good version of the Fuzz Face and acceptable version of the 808 if you flip the switches right. Besides, the Bixonic is a work of visual art and how can I pass up the opportunity to own something with "bi" in the title?
Early hard rock often featured a Treble Booster. Good luck finding one of those, but several builders make copies. Be sure to get one with a tone control or a switch you can flip. Divided by 13's unit has different profiles from which you can select.
Later, some guitarists, like Randy Rhodes, started using the "script logo" MXR Distortion +. For a while, the new ones didn't sound too good, but the one I bought last year is good.
My wild card for semi-clean tones, for Tom Sholtz, some EVH, and otherwise, is a Takky distortion. Its a high end Japanese boutique thing. I got it from Mesa Engineering. Often, if I'm not sure if I will need a pedal, this is the one I will take along. Possibly even a better all around, especially for single coil guitar players, is an overdrive called the Suhr Shiba. It has a mode switch and it can be controlled remotely by your guitar tech if you have one.
Many older players like Digital Music's Sparkle Drive, which allows you to blend clean and overdrive tones, or the Full Drive 2.
Digital Music also makes a version of the Jordan fuzz tone called a SuperFuzz. It can be too wild, but sometimes I use it for rougher Boston tones coming from Barry's Dirty Finger pickup equipped Gibson SG. One of the guys in the band had one of these guitars and its very evil.
I've been known to set up a Mu-tron on a dark setting and play side. It is an un-worldly sound as if George Harrison joined Sabbath.
Back when Ed came out, it was hard to replicate his tone. One inexpensive and acceptable metal solution is to take several pedals set at a moderate level and hook them together. I showed a this to a kid at Sam Ash using varies models of Digitechs. He said, "Wow!" The salesman walked over and said, "How the fuck are you doing that?" How do you think we did before modern amps?
Many of the players in my age group built their sound seemingly around one brand of pedals. For example, I'd guess Ed liked MXR and Slash like Maxton. I think Slash even used a Maxton EQ for overdrive. If you can figure this out, then it could be a real help at nailing the tone of certain players.
I don't have any metal guitars, so the Digitech Metal pedal is helpful.
At the risk of sounding like I went over the cliffs of Dover, the appropriate battery is important in these effects, because that's what they were designed around in their era. Instead of the old red EverReady batteries, you might try a lithium 9 volt instead as they seem slightly hot and last much longer. My understanding is there is a version of these in a smaller plastic case that you have to order. Meanwhile, get them at Lowe's.
Some overseas have complained it is too hard to get boutique guitar pedals. Well, if you live in Asia you might look into Takky or Moon. I just got a copy of an Orange Treble and Bass Boost pedal from guys called Arteffect in Israel.