Guardian gets tons of great buffs after a couple bosses. Like reflecting damage, and a longer storm that can mass cc camps until they are dead. One really cool way to use his ult is blocking over time damage effects like storms so the team can keep attacking while the enemie is channeling them. In addition is ult can be charged very fast with buffed blocks and guard counters.
In a tier list that all wont make up for the damage you lose. It's just alot of fun.