Leveling

First Class
For levels 10-25 gnats and other mobs at the beginning path of Rune Forest.

Levels 10-25 Quest level and get many many marks. Use practices for wisdom/vit/know to reduce reliance on specific leveling equipment.

For levels 20-45 idle farm hands, and peasant farmers outside Sigil, Rune or Tellerium

Levels 26-45 Tellerium mobs and farm mobs outside north gate.

For levels 40-50 boa constrictors in jungle northwest outside of sigil

For levels 50-60 (easy) lioness in wilds outside Sigil or Templeton. Don't forget to consider!

first class has Rune Graveyard too as well as Templeton which is also good for early 2nd class.

Levels 46-60 Templeton in a form. Very quick.

Second Class
For early second class level in Templeton (bag ladies, guards, foreign dignitaries, gangsters)

Also early second class level on plains killing easy lionesses (there are two kinds so consider) and hungry lions. Steer away from starving lions and dangerous lionesses (also looks like a lioness).

For second class Diocletian, Vospire, preferably in forms

Late second class (100+) hawks at the foot of Mount Copan (ne of Templeton) give full experience and can be easily soloed. However, it is best to have scan for these mobs.

Late second class, mobs in the basement of Templeton castle.

Late 2nd class, maybe 3rd class has Seelie castle. Good gold too. Just stay away from preservers.

Levels 61-90 Sleeping chambers in Sigil.

Third Class

For early third, kill bumblebees and merchants in Dio. Scholars in Dio give good exp until 160. Bumblebees and Merchants stop giving good exp before that.

For all of third and early fourth, alevelling with krakens and crocodiles, ayara between rune and rigel, the lake north of Dio (Bonns). Kill the easy krakens (use consider).

To make money while you level: My two favorite spots were the Druids south of Lowangen and the centaurs North of Tempelton. They are in the patches up there. It is good for Early Third.

Middle Third (160-175), mountaineers nw of NR. Avoid the yetis nearby by wearing a wizard cloak. Mountaineers have good gold.

All of third class, irda caverns on irda isle in a form.

I've also heard that rangers in tundras north of templeton were good for 3rd class

For late third class, kinsmen in the desert on the Beltane continent (ne part) are good.

Fourth Class

For early fourth class you can kill crocs in lake Ayara (outside NR) and Vospire Castle mobs.

For fourth class templeton catacombs(down from holy order temple), best in forms.

For late fourth class (the easy) nameless wanderers on faerie plane.

Another place for 4th classers is in the underworld near Decara in the wilds. Just beware of the white V's they are CPK. Some of the mobs are the okapi and the turtles not sure the names of the rest.

Also, in late fourth class, since I had a Decara house, I killed a lot of Drow sorcerers right outside the gate. Basic strategy for them is to dispel, blind, spell them up as good as you can. Slow helps a lot, as does a well-managed toy soldier. They give a lot of gold, and max experience right up to the bitter end. Easier undergroung mobs include okapis (which don't give full exp by the end) and turtles. Lionesses aren't bad if you can blind them.

Ohlmin's Guide
Let's see.
I'm strange this way - whenever I make a character, nine times out of time, it's a fighter first class. (Usually paladin. Cough.)

And yes, I do know that Ohlmin was a wizard first class, but I didn't level at all until level 47 or something.

Anyway. Cavaliers and paladins both get dodge at level 20, whereas valkyries get it earlier at level 15 and barbarians get it late - around level 43, if my facts are correct.
Once dodge is acquired, I usually immediately hit Templeton and level to my heart's content. After the first couple of levels, the rest go by extremely quickly - especially if one can adopt oneself into a form.

Second class, I'd say that Templeton remains good until about level 75 or so. Overseers give good experience and such, but I'd really recommend moving to Diocletian around level 75-80 (just remember to bring your wizard's cloaks).
Diocletian usually works to 120.

Post-multiclass, again, stay in Dio for 10-15 levels - some mobs still give 45k per kill for a while. After that, I'd say either hit Ayara or any of the other kraken lakes (there's one on every continent, really - Beltane has npk, of course, but there's still krakens there) for a few levels until you're 150 or whatnot, or just progress immediately to Irda caverns and level there.
At 175, hit Ogre. Trust me on this one. ^_^

Early fourth class, stay with ogre for 10 levels or so, or if you're finding that a trifle difficult that early in a class, kill krakens (45k still) to your heart's content. At some point (190, 195, 200, somewhere around there), head to the catacombs.

When I was leveling Ohlmin around 235, I tried Faerie once. I found that I personally leveled faster in the catacombs than off Nameless Wanderers - if only for the simple reason that there was ALWAYS a form going around the catacombs, and I didn't find a single one in the two days or so I spent in faerie.

That's all I can think of, really.