A lot of very good advice has been given... I don't think I can even add to it.

So I'd like to say this: the different modes play very differently. Whereas the Feast is arena styled and kill based, Frontlines are battleground styled and more objective based. This leads to the different playstyles that are needed for what it is that you're running. Don't be disheartened if you aren't great in one mode, try the others and see if that suits your style of play more.

I, personally, am god-awful at 4v4 Feast, yet I can be much better at 8v8 and Frontlines. Depending on the role you play and what your strengths are, you'll have more fun in whatever your playstyle suits.