Well this depends on your configurations, ship, preferences, and time.
For example, many players find it more profitable to box in the battlemaps than to kill aliens. If you can kill a bk easily by yourself, that's a great way to make uridium. Killing aliens in 4-5 is also good, but depending on your strength you may need to bring help, or only shoot the smaller ones. If you can get a good cubing group, that's a very good way to make uri especially if you have somebody considerably strong in your group. If you are in a clan, ask your clan members for help with any of those things, a good clan will have plenty of people willing to help.
I would say days of Boxing if you have the time. Save up for Galaxy Gates, (the harder ones pay better uri). I'm currently saving for Delta and Epsilon. Once those are completed, the rewards will very much be worth it!
My 2 cents worth? Its not about earning uri but saving and using it wisely. Use credits (because they're much easier to come by) rather than uri where you can. Buy your Goli and Irises for credits in Trade -- it can save you up to 347,000 uridium!! Other items like P.E.T. 10s and formations which are uridium only, so again, weigh their necessity against cost. Once your ship has ALL LF-3s and BO-2s, then consider the fluffy stuff. Plan ahead and play smart. Enjoy!