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Crown: This is extremely useful early on (Natural Armor bonuses can be difficult to come by at low levels), and a great way to get some mileage out of the Psychic Meditation feat prior to entering Meditant. It’s like a free Barkskin potion at level 1 that refills itself every day, lasts longer, and that you can activate without digging it out of your bag or drinking it, actions which would both provoke. Later, it stacks with the amulet of natural armor you're likely to have (thanks NineThePuma) meaning it can stay useful at mid-high levels.
Its main problem is coverage, similar to the Throat and Base centers, but having more AC is seldom a bad thing even if you have trouble keeping it on all day.
Intensifying it is unlikely to be worth your activations; often, Third Eye is the better choice defensively as you can pick up miss chance or a temp HP buffer that way via your powers. Not to mention, it does not add to your touch AC. But if you do want a more defensive meditant and don't care about the stat boosts (read: save DCs) then by all means use this one. It also can't be dispelled.
Usefulness: Feat only, IM, DIM.
Third Eye: This is perhaps the most universally-appealing basic center, particularly for PP-starved Psywars. You can never have enough PP, and since you get a fresh set of temporary points for every activation, this is a great one to fire off repeatedly throughout the day. You can even store them in cognizance crystals indefinitely (note that by RAW, only YOUR reserve returns to normal when the duration expires) or simply Bestow them targeting yourself or another manifester in the party to refill a permanent reserve. Or even just spend them on longer-term buffs. The possibilities are nigh-endless; just be sure you use them all, because they don’t stack/accumulate.
Intensifying this center causes the PP you gain to scale somewhat faster with your ML. However, I wouldn’t bother Intensifying it; at level 20, IM gets you a mere 5 additional PP (1.25*20 = 25) while costing two of your activations (2.5 extra PP per activation: compare to using those same 2 PP to simply trigger it again for a full 20 more temp PP instead.) DIM is little better, giving you 10 for 3 (3.33 per activation) and requiring you to tie up an extra activation in awakening. So I’d say leave this one basic.
My preferred strategy (especially at later levels) is to focus on other buffs, and put a leftover activation (if you have one) into this. While an hour of the other buffs at the wrong time or the end of the day can easily be wasted, there’s never a wrong time to have more power points so long as you have ways to use them. (Throw them in a Cog crystal, heal everyone up, make some quintessence, poison your weapons, do some divinations etc.)
Usefulness: Feat only, IM, DIM
Throat: Untyped bonuses to mental stats are very hard to come by, and you can get up to +6 to each of them - This is like getting 18 Wishes(!), albeit less permanent (without continuous reactivation anyway.) This comes into its own later in the game, particularly after you get Inner Harmony and can keep it up all day. Definitely one to prime at later levels.
For the most part, you'll only care about the one stat pertaining to your class, but it's nice to have this up for certain skill checks.
Usefulness: Feat only, IM, DIM
Heart: This one doesn’t impress me. Temp HP are nice and all, but you can get WAY more by picking Third Eye and simply casting Vigor (5/ML instead of 2/ML.) An hour or more of Fast Healing would be nice, but the only way to get that is to tie up precious activations priming it. Finally, by the time you get access to them, the Fast Healing 1 and 2 just aren’t going to be worthwhile except outside of combat, when you have plenty of other ways to heal. I would skip this.
If you got the Fast Healing - even just FH 1 - without needing to prime it, this would be fantastic, like carrying an eternal wand of lessor vigor with you every day, especially if you can Empathic Transfer everyone's wounds onto yourself.
Usefulness: Feat only, IM, DIM
Solar: Bonuses to saves are nice, especially since the Meditant only has one good save. The problem here is that the bonuses are typed (morale, which are easy to get - e.g. having a bard, or Heroism potions) and won’t stack with any other source that provides one. If for some reason you don’t have a way to get a morale bonus to saves then you can give this a look, but you’re better off with another center once you do. Also, it doesn’t scale well.
Usefulness: Feat only, IM, DIM
Base: The physical score buffs. Even pure manifesters will like this for the boosts to Con and Dex (HP, saves, initiative etc.) Gishes, naturally, will want to prioritize this even more, even activating it prior to entering Meditant for the useful boosts to attack, damage and survivability. Like Throat, this gets much better once you achieve Inner Harmony.
As this one governs concentration checks, two saving throws, and various other benefits, it's almost better than Throat even for manifesters. (And if you use no-save powers exclusively, it IS better.)
Usefulness: Feat only, IM, DIM
Sacral: Power Resistance. With ML boosts, this can get pretty high and last much longer than the power of the same name. Still, if you’re going to use it, best to go all in or not at all.
The feat version gives you perhaps the earliest non-race access to PR in the game, but it is ridiculously easy to overcome. (1d20+1 vs. DC 6, or 75% chance to beat it.) Still, that’s 25% “miss chance” for spells at level 1 and can potentially save you from that shocking grasp or sleep, even if it does rapidly degrade with level, so you can think about using it that way.
Usefulness: Feat only, IM, DIM