Who do you like most against rayquazza: articuno, lapras, jynx, cloyster, or walrein?

Rayquaza and Dragonite.

Depends on what you're trying to do. If you're trying to do very small-group clears, Jynx is probably necessary. Articuno is the best all-around Ice type currently, but Mamoswine (which is incoming) will beat all of these other contenders in all likelihood. I think it's ill-advised to power up any Ice Pokemon at this time.

On the other hand, Rayquaza handles himself pretty well (despite being glassy) and Dragonite is almost as good. Dragon types have more general utility than Ice types and aren't going to be immediately outclassed anytime soon; even if they are, both will still be good options.

EDIT: If you insist on powering up something Ice-type-wise, Piloswine is probably your best bet. It's a serviceable Ice type in its own right and does well against Rayquaza with access to Avalanche, and the dust won't be wasted because you can just evolve to Mamoswine when it comes out.

Articuno, hands-down. It has comparable bulk to Lapras and Walrein but with better DPS (and access to non-legacy ice beam) but with better DPS. Jynx beats them all in DPS, but is way too glassy for my tastes. The only downside to Articuno is that it's legendary, thus requiring more of a rare candy investment unless you raided hard on Articuno Day.

That said, I would wait until Rayquaza's return is a confirmed thing. If we should happen to get Gen 4 before Rayquaza's return, Articuno will be facing stiff competition from Mamoswine and possibly Glaceon and Weavile as well (depending on the moves they get, of course).

At present, Articuno

In gen IV, too many possibilities

I'd stick with articuno at present, as it does have high DPS and DTO agaisnt rayquaza, even against ancient power, and it also have versatility against groudon, and giratina, not to mention raids like alolan Eggtree, Golem, Rhydon. Jynx has more DPS, but its is about as fragile as it gets.

However, with gen IV on the horizon and Mamoswine, Weavile, and glaceon all with great stats and moves, it would be best to wait until a rayquaza announcement happens before doing anything, just save your resources for now

Wait to see if Gen 4 comes out before Ray's return. Mamoswine, Weavile, and Glaceon all currently stand to be better than any ice type currently in play depending on what moves Niantic bestows upon them.

Should Rayquaza return first your highest DPS options are Jynx, Piloswine (who's weirdly missing in your OP), and Articuno in that order. Keep in mind from a pure DPS standpoint that DT/OR Rayquaza beats all except FB/AV Jynx, and DT/OR Dragonite beats all Piloswine.

My Rayquaza team is 2 Lv 40 jynxes, 2 lv 40 rayquaza and 2 lv 40 dragonite. The thing is, for the most part the others are right, Gen4 should have some good ice types. We'll have to wait and see what moves they all get but since Rayquaza isn't a raid boss now I'd just keep saving the candy for all the above. Decide what to power up later.

One thing I will say, anything with blizzard (walrien, lapras, articuno) is a bad choice vs Rayquaza. I found that one-bar charge moves are too much of a liability. All of Rayquaza's charge moves are multi-bar and they hit hard. Doesn't matter that those three have more bulk if they never get off that second blizzard. My duo partner dropped his max walrein from his team for piloswine (until he got enough candies for another ray) because it was so risky.

Arcticuno definitely

Dragon types.