Define "Best".

I would suggest you look at the 50 gal hybrid condensing units. They are a cross between a tank and a tankless.

"80 gal won't cut it with 100 gal tub" -- if you have a 140 degree water and a 105 degree tub, it should be fine.

Last point: A 100 gal bathtub wastes a lot of water. Chances are that you aren't going to fill it very often. A 100 gal water heater doesn't make any sense for a 100 gal bath tub that you aren't going to fill often.

Get the hybrid 50 gal water heater that will save energy all of the time, and still be able to fill your tub. It probably won't cost any more than the oversized water heater.

Thanks ! Great points. Thats what I am considering right now.. a hybrid unit.

With the 80gal / 100gal tub analogy, I was assuming the guest suite would be pulling hot water too, dishwasher, etc.

And yea, you are right, we may not fill the tub everyday, but I would rather oversize and eat the excess energy costs than to completely deplete the hot water and run into temperature problems elsewhere every time we fill it.

Thx for the help !!