Best analogy:
DL/UL Speeds: How fast you can talk to me.
Ping: How far apart we are standing, and how quickly I can hear and understand what you're saying.
Packet Loss: You talking to me on a walkie talkie that keeps cutting out and I have to ask you to repeat yourself, this is the most crippling thing of all in online games.
They're both important, but ping is definitely more important. Games in general require very little actual bandwidth, but they need super fast communication (people hogging your bandwidth slow you down because there's no "room" for your requests left, we're talking milliseconds here).
A strong DL/UL speed is necessary to facilitate a good ping, but ping eventually will decide your online fate.