If you're flying United a lot of the time, I would think a United card would be the best fit. Free checked bag, priority boarding, United Club passes, 2x points on United flights...seems simple enough. The only problem I see is that right now the signup bonus is 30k for a $1k spend, and this usually goes up to 50k from time to time.

Chase Ink or CSP would be a solid option too being they transfer to United. Both are great long term cards if you plan on keeping them open for over a year, and both will earn you points you can use for United much faster because of their better bonus categories.