I've found Lathinlind is the easiest, if they haven't become a force that's expanded far. Their fiefs are scattered and their lords don't respond to the Marshal effectively due to the large split in where they are located.
Another good choice would be to take on the Engles. They are strong, but if you start with their port town via sea siege, you can easily take them down. My advice before doing this, build your Refuge next to a city you intend to take. Store up as many soldiers as you can into there (best to get it to level 3) and then hit the intended target with as many units as you can hold. Don't try to starve them out. Siege them out, take the city by force. Store up your garrison into it, and then withdraw all your refuge troops, bunker down into the city.
They will come siege that city. Cities are very valuable. I've always gotten 2-4 lords come knocking at my doors just a day or so after taking it. If you can, recruit from the local areas after the siege. Keep that garrison heavy and your own troops heavy.
The idea here, is to let them all come to you and try to take the city. Beat them down, and hopefully take some lords as prisoners. Once you have taken the city, beaten back their forces, you start to damage them. Raid their fiefs. Hold their lords captive. Attack traders, farmers, peasants, ect. Siege castles. Plunder them and leave them as theirs if you lack the numbers. Make the lords hate their current king/ruler. Eventually, you'll be able to make one or two lords defect. This is where the plundering and devastating their fiefs comes in handy. The places will hate you, but not the lords you give them to. It's a very difficult process to carve out your own kingdom from nothing in VC. There are many kingdoms, and many lords, stretched over a wide space. Prepare to invest time and a lot of money into rebuilding your armies and playing defensive, barely clinging onto what you've taken.
