Okay, so Vegas is an exceptional place for a little hot tub action, almost all the hotels have hot tub suites so it really comes down to view, budget, and atmosphere.
I do have to say the Cosmo suites are some of the best in the city, hands down. This hotel has went all out.....the BATH TUBS are really stately, large, angular. They are not bubbly, but they are soak worthy. They do not come with a view, as the bathroom is tucked away deep into the room, but they are big, rectangular, and fit more than one person. $$$$
Vdara has classic, streamlined, elegant, contemporary BATH TUBS that do come with view.....the rooms I've seen have floor to ceiling windows evening in the bathroom (we had our latest photoshoot there so the bathtub you see in Megan Fox's profile is the exact same bathtubs as in those rooms) It's nice, not sure if you can "lay out and relax" in there, but that might not be what you had in mind anyway. $$$
Now the corner suites in the Luxor have HOT TUBS that are elevated and in a corner of the pyramid. This is downright cool....It is an older hotel, but their hot tubs are bubbly, big, and fun. $$
Same as New York New York. Their hot tub suites are exposed in a corner the room. They're big, they are classic, and they are jetted. Some what of a view as these too are out in the main area and not in a separate room. Great fun for more than one person. $$
Wynn is nice, their tubs come with fabric pillows for your head and bathsalts to add.....they're big and you can watch TV while soaking. No views though.....$$$
Venetian and Palazzo, same thing, classic, easy to soak, nice tubs.....amenities are great, artwork is fresh, and they too are tucked away into the bathroom deep in the heart of the room. $$$
Those are the rooms that stand out in my mind. I have not been in every room in the city, nor have I seen the ones at the Mandarin so I cannot say, but if you want to make a night out of it, you might want to consider asking the reservations assistant if their rooms have "tubs with a view" ...if you are going to hit the strip then getting a view is definitely worth the consideration.