ok, here comes one hell of a long post:
normally if you want to go as a healing class, you have three choices for a primary: monk, elementalist, or ritualist. in certain situations, a necromancer can also handle heals well thanks to soul reaping, but due to the conditional energy gain of soul reaping and the effectiveness compared to a monk, its not the best option.
monk is generally perceived to be the best primary healing class, due in part both to the passive benefit of divine favor and access to monk runes. the monk attribute lines have access to almost every form of healing, protection, and support you could want from a primary healer (single target healing and team-wide healing, regen healing, single target and team protection, hex removal, condition removal, as well has "hard" resurrect skills, ie not rez sig).
the only thing they lack within there own lines is dedicated energy management (not counting signet use or simply knowing how to conserve your energy, both of which make for great energy management though). for a very long time, a mesmer secondary was seen by some to be a necessity, or at least desired, for a primary healer monk. however, the inspiration line of the mesmer has suffered several nerfs over time, the most recent being the changes to mantra of recall. because of this, many monks have turned to other secondaries looking for other benefits (assassin for evasion/dmg reduction skills, elementalist for glyphs). the inspiration line still has several valuable skills for energy management. however, for the most part, you can find most of what you need in the monk lines without having to investing in secondary class lines.
as for the elementalist, they can make a fine secondary or support healer. access to the energy storage line as well as glyphs gives them the ability to sustain their heals for a longer time than a monk. the problem arises in the strength of their heals. without access to monk runes, elementalist are limited to a max of 12 in healing/prot prayer, limiting their healing power. plus the lack of passive bonus of divine favor further reduces their healing power. where the elementalist shines is as a support healer. using the skill ether prodigy, you have the ability to spam skill like heal party, which is one of the most powerful heals there is. at 12 healing prayers, it heals each party member for 67 points: with 8 team members, thats 536 points of healing. spamming heal party helps take pressure off the monk(s), thereby increasing the survivability of the team, hence to term "support".
the ritualist, while neglected, can be very useful as both a secondary and even primary healer/protector. using the restoration line, the ritualist has access to many powerful and efficient healing skills. many of the better skills are dependent on meeting certain conditions, such as holding items or being near a spirit; this can make them less effective under unideal conditions. also, this line focuses more on smaller, straight heals rather than large spike heals or protection (aside from some of the weapon skills), so its sometimes seen as less favorable to the healing of a monk. i personally find the ritualist to be an adequate healer who has the ability to sustain his heals for a longer time than a pure monk. but the power of the heals, lack of protection, and, most notably, no hex removal, makes it hard to argue a ritualist over a monk.
i do, however, like the combination of skills such as attune was songkai, life, spirit transfer, spirit light, soothing memories, and mend body and soul. this give you several spammable straight heals, condition removal, powerful spike healing, as well some of the best energy management available. id put a build incorporating these skills up against a pure healing monk most of the time (might get flamed with this last part

).
the ritualist also has access to the communing line, which offers several very powerful protection skills. although much stronger prior to one of the recent skill balances, a communing rit, sometimes called a ritual lord, after the skill originally used in the build, or the more generic spirit spammer, still has the ability to offer all team mates in spirit range some very nice protection. using the three skill shelter, union, and displacement, rits have the ability to reduce dmg to the team to no more than 10% health, then reduce that dmg even further, all while adding a team wide evasion ability. since the skill balance, the rit lord has almost completely died out, although the build can still be used to some effectiveness, although not nearly as strong as before.
i could say some more stuff, but ive alrdy wasted too much time on this. may be more to come at some other time.