From the perspective of a trials healer, the worst build to heal is anything using a bow, because bow users tend to not stack properly because of the minimum distance on their AoE.
The runner up is melee Sorcs. I hate healing melee Sorcs. I have no idea what it is, but it seems like they take more damage than anything else.
Edited by timidobserver on September 12, 2014 5:05AM
V16 Uriel Stormblessed EP Magicka Templar(main)
V16 Derelict Vagabond EP Stamina DK
V16 Redacted Ep Stam Sorc
V16 Insolent EP Magicka Sorc(retired)
V16 Jed I Nyte EP Stamina NB(retired)
How about a "Paladin Templar" in full (or at least 5-piece) Heavy Armour, with a 2-hander, trying to mainly use weapon skills, Stamina, and pretend to heal a bit too.