The hell is going on in this thread? /sigh.. Let's look at every merit individually. It's not hard to do it logically. (With Abyssea as a main focus, and "other stuff" as a secondary.)
Group 1 Merits
Convert: Convert is an amazing ability that USED to be restricted to RDM. If you aren't putting merits in this, you may as well just go sch/rdm and be superior at.. everything. Seriously. Stop sucking and Merit CONVERT. Period.
Fire Magic Accuracy: Potentially useful with Fire IV around the corner. Addle is very accurate, so these merits are worthless for it. Worth the merits when Fire IV hits, even if temporary.
Ice Magic Accuracy: Pretty typical. This is most likely going to be our final merit of choice when Blizzard IV hits. One could argue Thunder IV being superior with those merits, but I would personally still take some added Bind/Paralyze. (Several abyssean NMs can be bound, and powerful binds do not break even after several nukes.)
Wind Magic Accuracy: Top dog at PRESENT, with our highest nuke being Aero IV. While several targets can be Grav'd, it is so few that one should not choose these merits for that. Likewise Silence.
Earth Magic Accuracy: Move on, this is always junk.
Lightning Magic Accuracy: As noted up in Ice, this may be a better choice than Ice when Thunder IV is available. Personal preference/Magian staff choices may dictate this being a better choice over Ice.
Water Magic Accuracy: Good merits. No. Really. Ever tried landing Poison? Fscking Poison I mean GOD DAMN. If anything, merit this if you are just pissed off at POISON NOT LANDING WITH SABOTEUR. (Note: If you don't complain about poison not landing, you aren't playing RDM properly!)
Group 2 Merits
Dia III: Somewhat outdated. Nothing in the game outside of perhaps AV requires a zerg, and that is most of what these merits are good for. If you do a lot of work with melee DDs and feel that an extra -5% defense on your target is worthwhile (So it dies faster and you can spend less time supporting them. Freaking needy DDs.) then pick up 1/5 of this. You won't have MP issues, and if DDs are on your target TP feed is not an issue. Just cast it more often.
Slow II: This is what makes a RDM better than a SCH at enfeebling. 4/5 or 5/5 this, or go level SCH. Seriously. They have more m.acc with landing Slow than we do, so maybe we should at LEAST have a more potent one? 4/5 Slow2 with SAB will allow capped Slow with Battlefield Elegy, 3/5 will not. Against an elegy-resistant target (There are a few), 4/5 is infinitely more useful than 1-3/5.
Paralyze II: I hate this spell. Placebo effects are just stupid. However, EYEBALLING says that it is worth at least 1/5 merits. Because I have not yet found good data on higher merit levels of this spell, I can only call those higher levels to be more placebo. Besides, it is always nice to overwrite Para1 from a weaker RDM in the MND department.
Phalanx II: Outdated. Not worth merits in any situation. If you want Phalanx on another person, you should be /sch to do it.
Bio III: Hot topic with Abyssea these days. Is it worth 5/5 anymore? Short answer is, No. Inside of Abyssea, the attack down is wonderful - but MP is plentiful, so just get a lower tier and recast it more often. The only situation 5/5 is useful are NMs you want to completely TP-starve, in which case.. You may not even want to cast Bio at all. Tier IV nukes are far more efficient Damage/TP ratio wise.
Blind II: Long overlooked, this merit may be making a comeback. (Doubtful, put more merits in Paralyze II, but first hear me out.) As a RDM, or many other jobs that aren't lolPLD, Abyssea NMs do NOT have capped hitrate! This means blind does matter. Also! Blind II is just an extra spell for trying to trigger grellow !!s, if merit spells are even an option for monster weakness. It may be worth 1/5ing, or more so if you work with evasion tanks.
Short version
Group 1:
Convert 5/5
Lightning/Ice 5/5 (Aero/Fire 5/5 if you want to move them around to fit our top nuke)
Group 2:
Slow II 4/5+
Bio III 3/5+
Dia III 1/5- (Preference, get it if you work
for with DDs)
Blind II 1/5- (Preference, potentially useful if triggering !!s)
Paralyze II X/5 (X is whatever you didn't put in slow/bio/dia/blind)
EDIT1: Updated Dia III section, changed potential final build.