HST, HTT, and HR are all gonna be big contributors but even if they are your top three your single target heals shouldn't be far behind. Surely it'll change per fight and depending on your healing comp but I rarely have any one spell account for 20% or more of my total healing, meaning my single target spells are in close contention for top three. Chain Heal is great when you can get the full effect out of it, but that happens under few circumstances, and when you can reliably get the full effect out of it you'll likely not want it glyphed. Chain Heal is quite terrible at triage healing because of its slow cast time and mediocre initial heal, two healing on progression you'll most likely find yourself using HS or GHW when it really matters. Variations in strategy will also affect things, our group is stacked for almost every Discharge, and the only other healing intensive part of that fight (Frostbite) Chain Heal is ineffective. Don't forsake Glyph of Riptide, it can be a game changer.
Honestly, there is a lot of really bad advise there.
1. Your single target heals should be nowhere near your top healing done in a 25 man raid. In 25 man, you should barely even be casting them. The only reason to cast a single target heal is (1) the target will die without one (2) HST, HR, RT are all on cooldown, and Chain Heal won't be effective on 3+ targets because the raid is too spread out (3) you are trying to maximize healing done during Ascendance or Ancestral Guidance. Single target heals across a full clear of ToT are less than 5% of my total healing.
2. If Healing Rain is not accounting for 25%+ of your total output, you aren't using it on cooldown or effectively. It should be 25%+ on stacked fights, and should be approaching 20% on spread fights. In 25 man, Healing Rain can and should be dropped on cooldown at least on melee. Even on heavily spread fights, you can almost always get it on 5+ targets (Dark Animus P1 is probably the only exception). Over the first 11 fights in ToT this week, my HR averaged 30% of my healing done.
3. You can get the full effect of Chain Heal the majority of the time, even on 25 man spread fights. Make sure that you have Vuhdo or some other addon setup to properly show you the amount of targets a Chain Heal would be effective on for each player on your raid frames. It isn't going to be a huge percentage of your output (like 10% on average), because it's a fairly weak spell. However, it should be your go to filler spell when ULE-HR, HR, HST and RT are on cooldown. 90% of the time, you can hit 4 targets in 25 man. I have yet to see a 25 man fight where the chaining glyph is worth it.
4. The Riptide glyph is awful and generally destroys your output by using it. The only exception this tier where it could be useful is Heroic Tortos progression (because overheal is so low, and it can be very difficult to hard cast on that fight). It reduces your HPM significantly, and on most fights, the Riptide HoT overheals for 70%+. Riptide ticks too slowly (it does about half the healing of Rejuv spread out over 18 seconds instead of 12 seconds) and too weakly to be competitive. Your output will be miserable if you try to use the Riptide glyph and start spamming HoTs. Even on fights with a lot of movement, you are generally better off dealing with the Riptide cooldown and making the best of your positioning and movement to get as much hard casting off as possible.
The 7613 haste threshold is nearly mandatory for 25 man raiding at 520+ ilvl, because that 7th Healing Rain tick is such a significant increase in output. Haste breakpoints outside of Healing Rain are generally not worth going for unless you are very close to them. The HST and HTT breakpoints continue to be bugged (they are affected by latency and can vary by several hundred haste points) and the Riptide and ELW breakpoints don't add enough throughput to be worth the loss of secondary stats in gearing to them.
For stat priority, I would go with.
Spirit to comfort > Haste to 7613 > Int > additional Spirit > Crit > Haste past 7613 > Mastery
Mastery is generally a completely awful stat in the current healing environment; the RSS addon has shown that I need somewhere around 3000 mastery to get 1% more output on the typical fight. I would take additional Spirit and haste past the breakpoints before Mastery - it really is that useless.
EDIT - I am just assuming you are doing 25 man based on the number of healers you listed in the original post, but realized that you didn't really clarify that.
if you are indeed 10 man i suggest you get the CH glyph because the riptide glyph is just bad and your single target heals are only ok. you don't need to use healing rain as much in 10 man if you can only get 1-2 people in it. single target spells are essentially your filler spell when CH, rip, hst, etc are on cd