Meditation: each point is 12 mana/minute.
If you harass a lot with mana in the laning phase, this is actually pretty useful. Someone on this thread said its useless even if you are trying to zone them really hard, because you will just conserve mana if you are running low... I say it is not useless, because you won't HAVE to conserve mana as quickly or for as long, which means you have a greater chance of zoning them more quickly/keeping them zoned.
Utility mastery: Depends on the champion... if you either benefit highly from the buffs, or will tend to get them often, 15% longer can be useful. If your champ doesn't use mana, this may also be your best bet.
Greed: Meh. If you are a mana user, I would almost always prefer meditation over greed. The only time I wouldn't is if you really really have no early mana issues at all. And I don't mean "oh, I just use my spells conservatively, so I don't have mana issues." Thats BS--you use your spells conservatively because if you didn't, you would have mana issues. Having meditation lets you use spells more often in the early game, and may help late game. (Even with Ryze & 5000+ mana, I've (rarely) run dry while out and about for a long time).
If your champ does not use mana, really really has no use for extra mana anytime in the game, AND doesn't really have any use from neutral buffs (or has minor use, but will almost never get them), then get greed. Sure, 6 gold per minute isn't that great. But it can help sometimes--if you are 15 gold short after a forced recall from lane (too low on HP, lets say), you woulda been ok with greed, which means you would have been out on the field quicker. You passively generate 7 gold per 5 seconds.
So, say 11.5 minutes into the game (no gold gain for first 1.5 minutes), you could have an extra 60 gold, and thus not get stuck after a recall. How long would you be stuck for? Up to 42 seconds or so, to generate that 60 gold. In fact, you probably wouldn't wait. 42 seconds is way too long.
So yeah, occasionally greed may net you an item you otherwise wouldn't have bothered waiting for, or may save you a bit of time, or may lower the time you have to wait to within acceptable boundaries. (Waiting 15 seconds instead of 40? Maybe you can would wait 15 seconds to finish that IE, but probably not 40 seconds)