Depends on what you want, but these are easy to level:
Steward - just by keeping max food variety and put some focus points, easiest leveling I believe, I remember having 200+ many times.
Smithing - extremely easy leveling currently because of javelins and 2h swords what costs 67k-110k.
Trade - kinda easy leveling if you trading a lot you'll get to 100 very fast.
Charm - easy to level by being merciful and letting all lords go.
Riding - I find it very easy to level in every campaign, you are very mobile and can run down fleeing for easy exp, not only in riding but weapons skills too. And horse archery of course is very easy and effective.
Bow, Crossbow - easy to level because you can keep distance and take no damage in return.(most of the time)
These are skills what usually end up at 1-50 even if I play a whole week on 1 character:
Medicine, Engineering, Scouting, Tactics, Roguery, Athletics, Throwing.
also leadership is hard to level in early-mid game.