I usually make separate tiers for separate positions (goalies, defensemen, forwards // QB, WR, RB, TE in football, for example).
In terms of goalie prospects, Murray is a tier 1 guy, and most likely the only guy in that tier (or even if a hypothetical tier 2). For example, a guy like Samsonov looks awesome, but he's so far down the pipeline, that his tier doesn't even register right now. Seeing as there are only 30 starting goalie positions, I would say that he is the most valuable of players out there.
When you consider forwards, look at Dobber's list: http://www.dobberhockey.com/?p=40669. If you set the prospect ranking cut-off for tier 1 at 90, then you have five guys in that tier (Matthews, Laine, Puljujarvi, Marner, and Strome). If you put tier 2's cut off at 75, you have another 7 forwards. In a 12 team draft, that means that you should be able to get at least one forward prospect who is "Vesey or better".
For these reasons, I would say that Murray is the most valuable prospect. If you cannot get him, some guys are starting to toot Saros' horn (Nashville) this off-season. With Rinne's injury history, he could be the next best bet goalie prospect to see any action. However, I wouldn't be passing on a high end defensive or offensive prospect to get him.