What pokemon to max out next?

I am level 37 now, and have maxed (to 39):
Blissey ZH/DG 96%
Dragonite DT/O 96%

Next in line:
Machamp C/DP 100%
Rayquaza DT/O 96%
Tyranitar B/Cr 96%
Kyogre Wf/HP 98%

I have Mewtwo (PC/SB), Raikou and Zapdos all at L35, and I'm not planning to power any of them further. They are already serving me well at L35.

Besides Gengar, none of those pokemons in your list are worth maxing out tbh. Even then, Gengar is only useful in niche situations, or T3 solo raids.
Useful legends are Groudon, Kyogre, Rayquaza, Raikou. Optional are Moltres/Entei, Lugia, Latios, Zapdos.

If the dilemma is between gengar and poliwrath I would go for the gengar. It make sense to invest in mons which are the best at a specific task. Gengar has the highest dps against psychic while poliwrath is a mid tier fighting mon. I haven't checked Pokebattler but I'm guessing a lvl 30 machamp is better than a maxed poliwrath.
I'm also lvl 37, I have maxed.
2x dragonite 96% and 91%.
Mewtwo 82%
Venusaur VW/FP 100%
Lvl 38 rayquaza 96% is on the way.

Well I have 4 maxed out Gengar and have almost maxed out my one perfect Poliwrath, so I'd say Gengar :). Poliwrath is tanky, but Gengar really helps against the clock in many raids.

Machamp is a better fighting mon easily. Harayama is also tankier and does more damage than Poliwrath. Double fighting Breloom and Blaziken are also better against the clock.

I'm not trying to hold you to the standard of scanner-users or those with access to fantastic biomes/nests, but I really wouldn't bother with most of those. A lot of those 'mons aren't the end-all of their type DPS so a bunch of level 30 ones, as most of those are, should serve you better. Maxing a mon should generally be reserved for those that outperform all other counters you may run into (For example, you may eventually get Tyranitars so I wouldn't max out Houndoom).

This means that Articuno is a pretty good candidate since its bulkiness puts it above Jynx and DPS puts it above Regice. Also, I would still work on the Mewtwo even at the costs and diminishing returns. Mewtwo has such high DPS that it can fill holes in almost any team with neutral DPS, and its two different type quick move and charge move put it high on the lists against many targets (those weak to ghost and those weak to psychic). With such a variety of targets, its important for it to hit bulkpoints to survive since its moves are pretty low speed, and breakpoints may differ with so many targets. I would continue with the Mewtwo - it's a wonderful multi-tool and in 13 EX-Raids I've never seen one nearly as good as yours.

If you had to pick one of the non-legendaries since you're accumulating Stardust faster than Rare Candies, I would go with Gengar, or work on walking a rare 'mon.