Well, as others have said, this is really based on your personal prefs. There are some factors, however that can influence this.
Out of all of the classes I have played, the one that I have had the easiest time with in getting levels is my cleric. The reason for this is simple. In the early levels a cleric can fight well and has good AC as well as some offensive spells and the ability to heal. I could effectively find places to solo until about lvl 20 if I wanted to. As we all find, in order to really progress in the higher levels you need to group. Clerics can almost always get a group. Healing is the key and healers get jobs. Period. And the way to really level is to group successfully. Clerics also offer very beneficial buffs. Grouping with a cleric, however is a role-filling task. You are there to heal. If you like to blast or melee, you can get yourself and your group in trouble. You heal, med, heal, med, buff, med. In smaller groups you might blast. You may also be the root master. I like the social aspect of the game so I am content to do this, especially in a good group. If you play your cleric badly, you will begin to have trouble getting groups.
With my druid I found the above to be true except that as a secodnary healer, you are not a first choice. You can show up and have people want you, but often they needed a true healer or tank, which a druid is neither. Druid buffs are valuable as well. Druids used to have value as a means of transportation but this is not the case anymore after the addition of the NExus and the Books.
In contrast, my warrior was the hardest to level, though I played him back in the first few months of EQ, pre-expansions. Could do nothing but group without having long periods of downtime. There were lots of other tank types so was hard to get a group. I made great progress when I ,latched onto a group of players and we played regular...but I went out of town on business for two weeks, came back and was too far behind all of my friends to get a group. I retired him shortly thereafter.
I have played to the mid levels several others...mage, wizzie and bard. All played easier than the warrior, but not like the druid and cleric.
Hope this is helpful. The first thing should be whether you enjoy playing the chracter, however!