Your spell list should change somewhat mob-to-mob, obviously.
In general, I toss Refresh as soon as it has cooled down (and assuming someone like a whm or pld, NOT a drk or blm, needs it).
In between and after Refreshes I generally lay down Paralyze, Dia II (Bio II once in a while, like when the healer can't keep up with the dmg), Frost or Choke (evasive mob, use Frost to lower AGI, tough mob use Choke to lower VIT), and Gravity, for the -5% eva.
Once in a very rare while I'll throw down something else like Slow or Blind (mostly when soloing or when lacking a pld tank).
Mage types obviously get regular doses of Silence (it unfortunately seems to get resisted quite a lot, even with a capped enfeebling skill and a Solid Wand (I wasn't 48 long enough to get a Rose +1, I dinged, went to help a friend with an AF1, got killed, and donged right back to 47), so that's a "do my best" kinda spell that gets an Elemental Seal in a pinch)
On the whole Dia (and Dia II) are FAR more useful than Bio (and Bio II) and if some idiot blm in your pt insists either on lvling their dark skill or that the two spells stack, calmly ask the pt leader to instruct him to not cast Bio, or find either a new blm or rdm, his choice (especially post 41 when you're in very high demand).
The elemental debuffs are really handy, and the only reason I don't use more of them is a matter of balancing MP use and the fact that I can never remember which stack, which overwrite, and which will have no effect in a pinch. I need to memorize that and the SC -> element correlation for every lv 1 and lv 2 SC, but have thus far been too lazy to.
As long as you have a blm sub, I strongly advocate using said elemental debuffs, even if there is a blm in the pt. In my experience, most have little to no understanding of debuffing or how stat changes affect a mob, outside of Burn and the -INT from that. So most will only cast Burn if they cast any at all, leaving you to pick up the slack. The additional side benefit of using them is that since they use your elemental skill, you can keep it capped or nearly capped (or at the very least at a respectable level) just by doing your primary job and debuffing.
There is no "perfect" line of debuffs, as every rdm thinks different ones have more or less merit, different party configurations require different debuffs, and some may or may not stick on a mob you're fighting, no matter how much you'd like them too (i.e. sticking Rasp on Spiders (earth based mobs) just isn't going to happen reliably on high-xp mobs no matter how much you dream).
As long as you use your head and judge whether a debuff will be useful or not, you'll be fine, no matter what you cast. Don't do dumb things like silence pugils to stop their "special spells" like Screwdriver (a TP move, which silence will do nothing to, and pugils don't have mp to cast with anyway). Use your noggin and really think about which debuffs are useful on a given mob, and which are worth your MP to cast. RDM requires more thought to play than just about any other job in the game, as you have more tasks to juggle and more situation-dependant spells.
For the record, I'm a rdm47/blm23, Mithran, with about 500 mp (changes based on weapons, and goes considerably higher with food), so I don't quite have the mana lake that taru have, but I have a fairly large supply. Even so, it is still painfully finite, so it takes some finesse to get the most out of my MP (only once, due to a massive clusterfvck have I ever run OOM before Convert was up (and often well past its 0:00). Pre-41 it wasn't difficult to run a couple battles in a row without resting by only using the debuffs that were absolutely necessary, and not just tossing random ones about for fun, and to say that I used everything in my ******* on a mob.
Amastacia || Asura server RDM 75
/ NIN 75
/ PLD 47 / BLM 37 / WHM 37 / WAR 37 / DRK 37
Windurst 10 / RoZ Complete / CoP Complete / ToAU Mission 33