The easiest faction in VI is no doubt the Vikings. They have kick-ass infantry with some uber units. Only their cav is weak but you will only be needing them against missile units and routers. The only problem for them is money, but raiding solves that.
It just gets a little dull when you're greatly outnumbered and you still know you'll win easily.

Behind the vikings are IMHO the Saxons, the Mercians, the Picts and the Irish. They all have their strongpoints and there weaknesses. Try to figure out what you're good and decide.

P.S. Never use a merc as general. Like you've seen they're not loyal to you and are easily bribed (and taking there entire stack). Secondly, mercs have sometimes questionable morale and the last thing you need is a general routing at the first contact with the enemy.