It depends on what exactly you're looking for.

Summoner: Utility comes in the form of a revival spell, a party regen, and a single target regen, and a debuff that weakens enemy damage that all the casters share; however, there are caveats here. Your party regen is not something you consciously use, but is a passive consequence to performing your rotation, and the single target regen can only be used during that phase of your rotation, so if you're looking to be someone who can potentially spot heal in a pinch, I don't think Summoner will really accomplish that for you. It can revive in a pinch, but casting Resurrection takes 8 seconds without swiftcast, and Summoner kind of likes using its swiftcast as a part of its rotation, which can sometimes make it tricky to actually pull off a revive when needed. Also, Physick scales off Mind and not Intelligence, so the healing will be entirely useless. It's a dead button. If you are interested in offensive buffs, it has a 2 minute party buff that isn't particularly strong but lasts longer than other party buffs.

Red Mage: Also has a revival spell as a form of utility along with a single target healing spell that you can use at any time, and a party-wide cooldown that reduces magic damage taken and increases healing received. The single target heal scales off Intelligence unlike Summoner's Physick, so you can use it to heal in a pinch, but you almost never will because it's just not that powerful regardless of stat scaling, and it will rarely be worth it to stop attacking to heal someone. Red Mage's dualcast mechanic, though, allows for any resurrection to be instant cast, making it a lot more effective at reviving. Having said all that, you won't really use these all that frequently, so if you're looking for the ability to regularly provide a little utility support, I don't actually think this gives you that much mileage. It's more like having some back pocket emergency tools. The magic defense and healing buff is nice, but that's about it for utility. It also has an offensive party buff though, mostly the same as Summoners. Generally, Red Mage is also the more fun caster on the DPS side of things and is more flexible, where Summoner's gameplay is very rigid and kind of on rails.

Bard: Both Bard and Dancer have a 10% damage reduction buff that is functionally identical, Troubadour for Bard and Shield Samba for Dancer. Beyond that, Bard has access to a single target debuff protection and is the only job in the game to have that. Debuffs that can be prevented or cleansed are not particularly common, but if you can remember to be aware of them, it actually feels really nice to use it. For example, in one of the savage fights currently, mistakes during certain parts can inflict Doom on someone that must be removed with either Esuna on a healer or Bard's Wardan's Paean, and Bard's tool is actually far more responsive, which is important because the Doom has a VERY short duration before it kills someone. Additionally, it has Nature's Minne which increases HP recovered for the whole party. In more casual forms of content, this probably won't feel very special or all that helpful, but in harder content I would argue it's far better than the flat healing that Dancer provides, because you can communicate with your healers to line it up with their stronger cooldowns in situations where healing gets tight. As for offensive buffs, Bard has several very weak ones it maintains constantly plus two party buffs every two minutes in the form of Battle Voice and Radiant Finale. I would say its offensive buff game is as prominent as Dancer's in terms of general feel.

Dancer: As I said with Bard, you have a 10% damage reduction buff that all physical ranged share, plus Curing Waltz and Improvisation. The former is a very small AoE heal that emanates from both you and another party member who you mark as your dance partner. These two AoEs do overlap, so when standing with your partner, it does provide a nice little bump of healing, and has a short cooldown. Improvisation is a small-medium sized AoE channeled regen that has a follow up which provides a small barrier as well. It's a channel, but you almost never want to stand still and maintain the channel and will usually try to double weave the regen and barrier between two weaponskills. It's not a bad action but it definitely feels very weird and clunky to use if I'm being honest. I'd say the healing effects you can grant will feel more effective in easier content--pretty much anything you use the duty finder for--and they still feel good in harder content, but don't have the same impact as powering up your healers. So it kinda depends on what you want out of your experience as to which utility effects will feel better to use on average. Dancer also has a small buff it maintains throughout the encounter on one person and yourself as well as a party-wide buff every 2 minutes and a critical hit/direct hit buff also every 2 minutes for your partner and yourself. Again, I'd say it feels about the same as with Bard.

TL;DR: If you really want the experience of providing a little extra healing often as you play through the game, regardless of how much your team is struggling, I'd say Dancer is your best bet. Unlike Red Mage, the healing doesn't interrupt your ability to attack and has a short cooldown that will let you help out with healing very often in anything from dungeons to ultimate raiding. It's also AoE whereas Red Mage's is single target only. Bard's buff has more potential in challenging fights that actually need extra healing, but that comes from making your healers stronger rather than providing extra healing yourself. Red Mage's main trait is its ability to revive party members quickly and easily, which Bard and Dancer cannot do at all, but you only need to revive people when players mess up and more often then not, you wont' really be providing much utility at all. And Summoner has the strongest healing of them all thanks to a trait that Dancer doesn't have, but its healing isn't something you really control, and will only get once every 2 minutes.