The picts

[matteus the inbred]

From the opposite 'Viking' side, I can safely say that the nastiest reverses I ever suffered as the Vikings have been at the hands of the Picts (the Irish can give you the runaround, but cavalry sorts that out). Very good defensive provinces, lotsa crossbows and some hard hitting infantry make them a nasty proposition.
In my current (although somewhat 'shelved') Scots campaign, the Picts took aaaages to subdue as their berserkers repeatedly made mincemeat of my Highlanders. Good faction.
As with the Scots, it'd probably be easier if you allied with the Vikings and built a lot of ships to prevent them betraying and raiding you, but where's the fun in that???