I'm not going to go into too much detail about "Classic" Diablo II. Don't get me wrong, I covet my experiences playing classic and it's an awesome distinct way to play the game (Can we all agree that classic whirlwind was awesome?). The issue is that the community is pretty much dead. In 1.11, 1.12, and 1.13 Classic was thriving pretty decent, but since 1.14 it's become a ghost town (You will rarely find more than 2 games at any given time and sometimes no games at all). There's no point in writing in-depth guides and whatnot about a game that no one is playing anymore. Perhaps Diablo II: Resurrected will change that, but I have pessimistic expectations for that. I feel like, while LoD will certainly see a revival, classic will likely remain a ghost town. If I happen to be wrong, however, that would be awesome and I might be open to writing more about Classic.

For now, I am just going to reserve this page solely for tidbits about Classic.

Rare items are the status quo

The most notable difference in classic is that yellow (Rare) items run the show, almost exclusively. Unique items have their uses and roles, but ultimately (Magefist being a sole exception) there aren't any instances where a rare with the right mod rolls couldn't outclass a unique. So if you look at the most elite character builds, you'll see that every single item is a rare item.

What does a good rare item look like?

Generally, what a good rare item looks like depends somewhat on the type of item, the build, and whether other breakpoints are achieved by the rest of the items. However, a noticeable pattern is FHR, Life, Resistance, Resistance.

For instance an example of a great rare armor for a sorceress would be
____________________
Dire Shell
Mage Plate
Defense Rating: 655

+24% Faster hit recovery
+150% Enhanced Defense
+47 life
+24% lightning resistance
+37% cold resistance
____________________

Then the owner of this armor would likely use a SoJ to socket it and insert a Perfect Ruby to add a total of 85 life, instead of 47.

Classic Staples

As mentioned before, Magefist is the only unique that is set in stone as a best option. However, many classic players would play budget on some characters and so there are a number of unique & set items that would be used.

Angelic Wings + Angelic Halo
The angelic ring & ammy combo is very common. The amount of attack rating it gives is unparalleled and the large life boost is a bonus.

Sigon Helm + Sigons Gloves
Not much in the low budget range can compete with a ton of attack rating, 30% ias, and a bunch of mana for melee characters.

Twitchthroe
Twitch offers pretty much everything a melee character needs except resistances.

Ume's Lament
The standard wand for necromancers.

Stone of Jordan
SoJ is perhaps the most important unique in classic. It has been used as currency, is often used to put sockets in items, and is a pretty good ring too, of course.

The Ward
Used when a 30/20 Grim Shield with tri-resistances isn't in the budget.

Bloodfist
Used pretty often on melee characters.

Gull
The weapon of choice to magic find with.

Spectral Shard
The weapon of choice for most low budget casters.

Bonesnap
The weapon of choice for low budget whirlwind barbarians and chargers.

Classic Whirlwind was different
LoD players don't often know that whirlwind was much stronger in classic. In fact, it's one of the top two best builds. The expansion gutted its hit-check rate (The amount of hits it can get in an attack) severely. When you whirlwind, you will hear a successful hit as a "tick" sound. In the expansion, if you whirlwind into a group of monsters, you will maybe hear 2-5 ticks. In Classic, there are so many hits that it makes a staccato sound. 

The overall best PvP builds
#1. Hammerdin (No surprise there)
#2. Whirlwind Barbarian
#3. Blizzard Sorceress
#4. Bone Spear/Spirit Necromancer
#5. Fireball Sorceress

That's really it. Chargers and Fisters have their place, but are more niche. If you were to take a time machine back to patch 1.12 when classic was most alive, 90% of the builds you would encounter would be the 5 listed. (You would also encounter the infamous "FC Zon" fairly often, but hacked characters don't count).

Low Level Dueling was pretty big in classic
LLD is pretty niche in LoD, but in classic, it had some popularity. Every ladder reset, there would be a large group of players trying to secure the "#1 LLD character" title.

I'm proud to say that I held onto that title for the first 6 months of 1.13 on USWest, largely due to an incredible cold-damage Naga I found myself in the first two weeks of ladder. I lost the title to a hammerdin eventually, which was inevitable.