Heighten Spell. It's a variable-tic MF, and boost that DC to the level you choose to bump up to.
For buffing spells, Heighten Spell has very limited value, IMO.
Your best option is probably Extend Spell, though you might want to consider getting a Lesser rod for 3,000gp to use it 3x per day for "free", rather than spending the feat and boosting the spell level from 1st to 2nd.
Since the warpriest spell list is mostly going to be about buffing and status removal (because your casting stat is a secondary stat as you're not a primary caster, and because the spell list you have doesn't have a lot of offensive spells on it) most metamagic just isn't good for you.
I agree with Steel Refrain that Extend metamagic is probably the only one you would be interested in, and that its probably better to buy a rod of it then waste a feat.
It's just that Warpriest don't do much offensive spell casting, or at least they don't do it very well.
Most metamagic is oriented to be offensive, not defensive so it just generally doesn't do you much good.
1st to 2nd?
Yeah, not too many on the list, other than just upping the buffs a bit.
Now, if we are talking about 2nd to 3rd... maybe. I know they have a somewhat decent blasting option with admonishing ray (which is scorching ray, but nonlethal force damage- less likely to face resistances which would severely limit the fire version, and you can eye ball what is immune with ease since it would be undead and such).
Obviously, blasting is a secondary concern (although it does decent enough damage). I value this since it is a standard action ranged option that is a force spell. So throwing toppling spell onto it seems like a great move to knock enemies prone when they are too far off for melee. That way, they either can't full attack you (since they wasted time getting up), or their attacks face terrible penalties and you get big melee bonuses.
If you are in a campaign with few nonlethal immune creatures, then I would even advise that one trait that lowers metamagic cost (which would keep it in the 2nd level range for this discussion). Loading your 1st level spells with buffs, and then 2nd level spells with admonishing rays. Seems like a nice, tidy little style. Heck, I might have actually used it in dark souls with emit force...
You could look at furious with a barb dip. It'll let you cast spells while raging
Silent spell & fervor lets you cast spells while paralyzed, which makes it possible to cast surmount affliction to get around that condition. Silent spell also gets around silence of course which may be handy if you're likely to fight spellcasters.
Since Surmount Affliction has both somatic AND verbal components, wouldn't that trick require both Silent and Still metamagic?
Seems too heavy a feat and level increase tax to make it worthwhile, I think. Having Freedom of Movement active for the same 4th level spell seems a better use of resources...
Too bad, as it seemed like a very handy trick!
Fervor bypasses the somatic requirement. It's still a harsh cost for a warpriest - you have to prepare a 3rd level spell to add just one condition to the list that surmount affliction can effectively deal with - but you may have enough feats as a warpriest to burn one on this very situational stuff.
I've played a great sword wielding warpriest up to tenth level so far in pfs. I have never felt the need to use metamagic feats for in combat casting. Extend rod is handy for out of combat buffs.
Reach spell, bump your touch healing spells up to close ranged touch (25' + 5'/2Lvls) for +1 Spell level. It's great for those emergency situations where you are just a bit too far away.
In my experience if I need that emergency healing just channel. Add to that the fact that the warpriest is wading into melee most times and he is the one that needs that emergency healing.
@ avr -- Nice call! Had neglected to consider Fervor in the mix. Agreed it is a hefty cost, but a neat trick for emergencies. Going to file that away...
Regarding using Reach spell for healing at range, it is also a bit of a niche case, and I agree with Mulgar that it is probably more easily surmounted with other options (for example, casting Grace and then just moving to heal someone). Might be more useful on an archer Warpriest, however, as they won't be mixing it up on the frontlines quite so much.
Personally, I'd still hesitate to build a Warpriest for healing purposes, unless it is out of necessity (for party dynamics, etc.).
The base warpriest certainly isn't a good combat healer. I'm surprised they don't have an archetype for that though.