I have stayed multiple times at all 3 properties including suites at each. It may depend on your tastes, but I would recommend the GH Seattle as the "best" overall for a romantic luxury weekend, with the O8 as a close second.

My take on the 3:

1) HR Bellevue - This is a 'nothing special' sort of city hotel. I find the rooms to be a bit ho-hum and in need of a refresh. In particular the worn brass door fixtures, the tub showers, and the almond switch plates really detract from any 'luxury' feel that might have once been present. Pluses: modern pool and fitness center, M in the Regency Club is truly a gem in Hyatt's portfolio, and suites do get upgraded brown bath linens (but still no escaping those tub showers!).

I have occasionally been upgraded to an executive suite, but this is out of the norm, and I find them rather lackluster in any case. It's pretty much a set of French doors that separate a comically small/awkward bed area from the rest of a normal room-sized sitting area with a couch. The single standard bathroom remains near the main entry instead of attached to the bedroom.

2) Olive 8 - Designed as a sleek/modern hotel, this property's LEED UltraTitaniumSapphirePlatinum status pretty much just means that your shower will be disappointingly anemic. Diamond upgrades will normally be no more than a corner king, which despite a published $50/night rate differential, amounts to pretty much just a weird unusable entry hall. They are fairly consistent about granting Nor1 upsell upgrades to suites (starting at $135/night if I remember right). For a romantic/splurge type stay, these are well worth it. The luxury suites are spacious and stylishly-decorated, and include a minibar/sink area in the parlor to have a proper night in.

Pluses here: diamond breakfast is complimentary even via room service. They always tell me I'll only pay the small delivery charge, but it's never showed up on my bill. The salt-water pool is a welcome treat, and includes a locker/changing area plus my favorite pool amenity: a handy swim-suit spin dryer so you'll never have to pack a dripping pair of trunks again!

3) Grand Hyatt - A more 'traditional' take on luxury; if the O8 is cold steel and glass, the GH is warm wood and stone. With no true on-site restaurant, breakfast options are limited to the club, but the sushi restaurant and steak house are convenient for a swanky dinner. They offer use of the pool/spa at the O8, but I find despite the close proximity it's always just slightly too much hassle to make the trek.

Diamond treatment here is excellent; with 30%+ of its rooms configured as suites, it's not uncommon to receive a complimentary suite upgrade at checkin. I prefer the executive suites which have a guest bath, front closet, bar with sink, and doors separating the parlor from the bedroom. The emerald suites have a slightly wonky layout which doesn't provide true separation between the rooms and don't use the space as well, but tend to have better views (ask for a xx22 room for the best views).

Overall I would recommend either of the downtown Seattle properties with a slight preference for the GH. I would not consider the HR Bellevue a desirable place to spend a romantic evening of any sort.