A lot of trinket power is based on the fight and timers.
(note that best is when you most out of the trinket, not suggesting that it's actually the BiS trinket for that situation)
An example comparison list:
660 PvP trinket, best when you have adds that spawn on a timer that follows this trinkets 35 second CD, these fight's include Brackenspore and Tectus. Good when you have a fight with irregular duration and requires off the CD use of incarnation & CA. The trinket has no base stats so it's extremely dependent on the player to make the most of it when the proc is up.
Sandman's Pouch: Best when you have a fight that has a 5 min or 7 min duration, and you can hold your CD's, or optionally when something big happens at 2 min marker and you can hold your CD's for that. Decent for all fights
Quiescent Runestone: All around strong due to base int, less optimal for Brackenspore, due to the inherent lack of burst AoE damage.
Shards of Nothing: Best when you have a single target only fight, use depends on other trinkets, fight duration and nature of the situation. Least optimal for AoE, but has strong base int to carry over this minor detail.
Copeland's Clarity: Similar to Sandman, has the same strengths, but a lot weaker when you can't utilize it properly, Strongest non highmaul trinket (unless you have 3/3 Sandman), but can't be combined with Shards of Nothing.
For single target overall rankings, you can refer to this list: http://i.imgur.com/L5LhiWq.png (credit to whoever made this, found it in the forums)
But in reality if you can choose, you should consider the fight it self.
Oh and you should also bare in mind that you only get real "value" out of your trinkets during CD's and AoE, so Sandman > Copeland if combined with Shards in almost any case.