Celts is a good start.
Good siege and the woodcutting buff are big benefits.
Mayans are the worst because their resources last longer.
My opinion is that booming in castle is a good choice.
Get a second and third TC, start to get your vil count up and definitely get your farm count up, then go up to imp.
Hussar/halb in front, onagers/heavy scorp in front and treb/ram as the kicker.
Spanish are also a pretty decent civ in diplo because of the trade bonus.
Portuguese is very strong because they have free stone/good income with Feitorias.