General Advice Leveling Advice
This guide isn't necessarily the fastest way to level but I've speed run to 101 so many times that this is coming from a person who is super set in his ways and just does things by force of habit.
If you can find mobs that haven't been killed in a while they'll give bonus XP. You also get bonus XP from grouping with other people and you can use mudscrabble points (HELP SCRABBLE) to buy EXP bonuses (for a certain period of time). Additionally, if you're good aligned you'll get more exp from evil aligned mobs (and vice versa) - this is just a generalized guide
Levels 1-4
Kill mobs in mud school and around Palanthas. Palanthas you want to kill kids, citizens, commoners, urchins, cats and dogs. Avoid clerics (who also have keyword citizen), guards, cadets, and enforcers who will attack you if you attack something in a room. Unless you have a ton of EXP required per level this should go fairly fast.
Level 5-14
Get the subterranean lizards out of the Palanthas Sewers. These act as charmies (mobs you can order around). There are 4 of them. To get them you give them food (ie: give pie liz or give pie 2.liz) and then group them (ie: group liz or group 2.liz). You have to be visible for them to follow you. The sewers are a little dangerous so it's a good idea to go invis until you find the lizard, go vis, get them, and then go invis again. You'll need to continue to feed the lizards over time otherwise they will no longer remained charmed and will wander home. While you are in combat with them - if you type "or all tail" they'll do tail sweeps which do large amounts of damage (you can also order them to "bite" and it will do similar damage).
For the actual leveling you can use the Palanthas Library (kill scholars, apprentices, readers) and Garnet (in the palace kill clerks, gardeners, and the captain of the guard) and kill cityguards outside the palace.
If you're good aligned Lemish gives good XP for the different dragonarmy barracks. This lasts until about level 17. You can also use the dragonarmy camp west of Neraka - this works until about level 20.
Level 14 is the last time that you can choose whether or not to be PK. If you are going to be PK you have to be more cautious while leveling.
Level 15-20
HCT 2nd level is a good option. Kill Priests, altar boys, office clerks, office workers, administrators, good minions if evil and evil minions if good. Library also continues to be decent XP with scholars, historians, researchers and peacocks. Caergoth's scruffy sailors give good XP especially for good aligned chars. Additionaly Lacynos gives very good XP for good align.
Level 21-30
Caergoth road has very few mobs - but they give very good XP. to evil/neutral alignment - Solamnic Warhorses are especially good, but patrols are okay too. Caergoth itself has scruffy sailors by the wharf which are easy to kill and give solid XP. You can also try Haven - lots of different mobs there that will last you for a number of levels. Good align can use Meitol for great XP.
Level 31-40
HCT Level 3 gives decent experience. Also, you can continue killing guards in Haven. Kurmost is also great for evil/neutral once you get to 35 or so. Meitol continues to be good for good aligned chars. You can also use Zhakar if good aligned. If you use Shaker you just have to watch out for the agro mobs, they're all easy enough to kill but there are often a couple of them in the same room.
Level 41-50
Kurmost continues to give good XP for evil and neutral characters. Good characters can start in on Hillhome, children are easy enough and townspeople/militia aren't much harder.
Level 51-60
Stone Rose is the best experience for these levels. Hillhome works too for a bit. These two aren't as great for neutral characters, but you can use Ahlanost which works well for neutral/evil aligned chars and the mobs are easier to kill.
Level 61-80
Stone Rose. It gets really grindy but Stone Rose has by far the best exp for these levels.
80+
Skullcap is the best here. It's hard to do without charmies, but if you are a caster or can glom onto one it's great.
High 90s
You can continue using Skullcap but it's slow going. I quest/spam skills for levels at this point. Languages are really great for this as they go up super quickly.