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I feel like each of the other options you list gives up more to get more, though, which I feel violates the "almost always better" aspect of sky-blue.  The picks you mention offer much tougher judgment calls - if nothing else, each one of these drops down to a single encounter heal, and are unlikely to pick up significant healing powers from their hybrid class to make up for it. 

A serene runepriest/swarm druid is probably the best example of the tradeoffs inherent in these options - obscenely tough, but suffers significant constraint on the use of their runepriest powers in Beast Form.  It needs its hybrid talent to grab Swarm Form's DR so it can't have Rune Mastery, and it basically can't ever hit with a Runepriest attack due to crappy Strength so it's pretty much a waste to try to grab an alternative runestate.  You're even a sub-par Druid, losing 2 at-wills (1 through hybridizing and losing the bonus at-will feature, 1 through having to take a Runepriest at-will that you can't use in Beast Form and don't have the stats to make effective) 1 encounter, and 1 daily (some of which you can probably find as non-Strength rolls, but your choice is pretty limited).  Being obscenely tough may make you the last person standing, but many parties would benefit from their controller more fully embodying that role than being nigh unkillable.  Awesome tank chassis (although still probably an off-defender due to lack of multimarking)... but nothing like a Runepriest. 

Edit: Avenger comment withdrawn.  Amazing striker, durable, probably a better Hammer of Vengeance than a pure Runepriest.  However, would have to be seen as mainly filling a striker role with some splashed off-leader options.   

My point is, if you want to play a runepriest for the things that a runepriest is good at (frontline leader), odds are you'd almost always benefit from hybridizing with Battle Cleric.  For the most part, you're like a runepriest, only better, at least for the first 15 levels of your life.  Only when you start using runestates that end do you start to feel any less rune-y, and if you do so you're probably picking up Channel Divinity for a whole lot of extra awesome (Healer's Mercy may be hard on your DPR but it's amazing for your party's health, and the hybrid can downgrade a move to a minor to launch Healer's Mercy, Rune of Mending, and Healing Word all in the same round).  Given that you'll want the crit range for Rune of Battle you might as well get an early start in Paragon crit-fishing with a Jagged Greataxe with Gambler's Word, which does nice things for your damage output and healing power.

TL:DR - For some trying to hold down the Leader slot in their party as a Runepriest, I think there's a good case to be made that hybrid Serene / Battle Cleric is sky-blue.