Background: the pages on skills in D2 are missing information post level 20. Some practical and philosophical thoughts on how to tackle the matter are in order.
Level 20 is the soft cap on any skill in D2. However this can be boosted, sometimes significantly. I need to figure out how high this cap is. On my current Javazon I have a total of +15 to skills (bringing my level on maxed skills to 35); I can probably still bump this up by at least about 6 levels in equipment alone, and then another +10 or so with skill-boosting grand charms. So we're looking at a hypothetical max of ~50 for most skills (assuming there's no hidden final cap somewhere). However, there are many problems to this.
The first one is that not every skill will have the same cap. Tearhaunch, for example, gives +2 to vigor--I'm fairly certain no other boots boost any auras, so in a hypothetical world where everything was fully tested we'd have 2 extra levels for vigor, which results in non-standard pages when we input the data. A fair compromise, I think, is to use the lowest common denominator. Another fair one, considering how difficult it is to get to level 50 for ANY skill on a non-hacked playthrough, is to just edit each skill page to level 30 or so for now and debate the rest of it later.
Just for fun (and maybe for future editing), let's see how high the number can go (legitimately). Set items and bonuses are ignored because even with set bonuses all of them are just not as good.
Helms: Unique Helms have a +2 skill modifier at most. Rune Word helm with highest skill is Delirium, also +2. Each class has one or two specific skill trees that can get +3 with a magic circlet or class-specific helm. Druids and Barbarians can get up to +6 in total of one skill thanks to class specific item bonuses.
Amulets: +2 to all skills for magic/rare/crafted/uniques (except paladin, which with Seraph's Hymn can get +4 defensive auras in total). Magic Amulets can have +3 to one tree.
Rings: SoJ/Bul-Kathos have +1 all skills, two rings=2 skill levels.
Charms: +1 from Annihlus, +2 from Hellfire Torch, +9 from 9 grand charms in a stuffed inventory that already has the Annihlus and Torch in it (I'm fairly certain putting them in the Horadric Cube doesn't count).
Body Armor: Arkaine's Valor gives potentially +2 all; Ormus' Robes gives +3 to a random sorc skill; Amazons get +4 Critical Strike with Peace rune word.
Belts: Arachnid Mesh is the only belt to give +1 to everything. Snowclash gives +3 Glacial Spike and +2 Chilling Armor/Blizzard; Thundergod's gives +3 Lightning Fury/Lightning Strike.
Gloves: Assassins and Amazons can have +3 on their gloves. Magefist gives +1 to Fire Skills.
Boots: Gorefoot gives +2 to leap; Tearhaunch gives +2 to Vigor; Marrowwalk gives +2 Skeleton Mastery; Shadow Dancer gives +2 Shadow Disciplines.
Shield: Spirit gives +2 all skills. Herald of Zakarum gives +4 combat skills. All two-handed weapons miss out on these bonuses unless you're a Barbarian.
Weapon: A doozy. Arioc's Needle gives +4 all, Mang Song's Lesson gives +5 all, and a Heart of the Oak gives +3 all. The first two are two handed, so Mang Song's is tied with Heart of the Oak when paired with a Spirit as highest overall boosts. There are a LOT of individual skills or trees that get pluses with other items, but as we're looking at the lowest common denominator here...
So. In total we have +2 helm, 3 to amulets, 2 from rings, 12 from charms, 2 from armor, 1 from belt, 2 from shield, and 5 from weapon+shield or two handed weapon. That gives us +29 skills, which means the highest for most skills to get to is level 49 in total. But what about the highest? It's too much work for me right now to go back and dig up every single skill bonus again, but for starters consider +2 helm +3 amulet +2 rings +12 charms +3 armor +1 belt +1 gloves +2 shield +9 leaf rune word weapon equals...55 skill points to a random sorc fire skill. That'd be quite a sight to see!