A dead healer ultimately doesn't heal... statwise it generally is:
Avoidance (e.g.: does not mitigate, but completly prevent damage AND statuseffects) >= healcrit >= else
Depending* on the format you aspire to play in and your class it is:
Wounds (e.g.: bolstering for undefendable morale dumps and otherwise semi-static [or armor/resis/wounds debuff-powered] bursts [BW, WH, Sorc, WL,...])
>=
Initiative (initiative debuffs are the most potent [most classes have disgustingly low initiative base valuees and access to few gear offering initiative] given the exponential nature of initiative, e.g.: if your ctbc postdebuff is >40, well... [on crit effects, i.e.: healdebuffs,... and crit multiplier]) and -ctbc (more effective on order than it is on destro, all things considered)
>=
Toughness (note: since the overhaul it directly counters your attacker's mainstat, e.g.: harder to properly debuff/circumvent than armor and resistances [stock** WE/WH/WL/Mara in terms of armor, stock BW/Sorc in terms of resistances], better for small scale given the significance of mainstat coefficients in casts/dots/channels; the rdps-circlejerk) - at best in conjunction with relatively high base mitigation [= armor/resis]
>=
Armor and resistances (e.g.: best returns against pugs, goodish returns in overall (read: pug-riddled) large scale ORvR [due to proper application of most potent debuffs being finnicky/not worth it - e.g.: if your targets are getting killed within a few seconds by virtue of the numbers in play, or are part of a dedicated 6man {e.g.: cleanses including on-use cloaks, kiting, ...}, high avoidance via HtL dancing tank crowds and other means {e.g.: IB parrybuff}], medicore returns against organized opposition [in small scale because of <proper counterplay> {e.g.: positioning and awareness, CC, guard swaps, detaunts and general coordination > easily debuffable stats} and assists - in large scale because of the numbers in play, morale dumps and assisttrains]).
*Accessibilty of stats and predominance of (de)buffs in the respective format and on the respective class(es in play). (I.e.: Depending on the circumstances [opposition/your composition and class/environment; map dimensions {e.g.:CQ SCs vs ORvR} and numbers in play] some stats can be neglected.)
**Stats that require the deployment of multiple toolkits to be countered; are countered by composition > stats that the most potent classes can counter on their own. (I.e.: You gotta factor in uptime etc; as more gcds and gear your opponents gotta spend to setup and funnel potency into their dps, the better your returns are - as easier the application and as higher the uptime of debuffs, the worse your returns; some stats - on certain classes - just cannot be stacked high enough for that reason; to make the investments worth it).
TLDR: There is no definitive go-to setup for all formats (avoidance stacking is king tho), your best bet is to build a proper group and dedicate yourself to it, or build your character around your format ~ don't expect to perform well in every format, unless you are grouping.
Note: The biggest increase in potency; the most tremendous progression is <factually> enabled by and achieved with a dedicated group; i.e.: a proper group in green gear, but with a properly crafted composition and coordination > BiS pugs with supposed 'cookie cutter' specs.