Ideally you would choose more than one imbue (if you can afford it). Vampirism tends to be a must among knights (strongly recommended to do t3 if possible). Thats followed by mana leech, so you can cast your rotations non stop while boxed without having to worry too much about running out of mana.
I personally think skills/critical is not as useful at lower lvls, but if you want to go for it, your damage will be about 3-5% higher depending on what you go for. If you have the money go for critical, otherwise do the skill imbue which is a little weaker but tends to be muchhh cheaper.