Doesn't seem to ever matter what orchids are in bloom, peoples' attention first goes to the bromeliads, even if they're tucked to the back of the collection. Granted, that's not one specific plant, so it's a bit of a cheat in terms of answering the question.
There are generally two aspects that attract attention. First, some people are immediately drawn to the "air plants", aka tillandsias, because the growth habit is so different from anything they are familiar with. The tills just spark curiosity in people. Second, most of the other genera I grow (like the crypts and neos) have very brightly colored and attractively patterned foliage year round that just seems to pop, more so than the parvis and mottled leaf paphs.
If I limit my answer to orchids, the plants that people notice first are the phals. Not any specific one, just whatever happens to be in bloom at the time. I have enough of them, and enough variety, that there's usually at least one in bloom at any given time.
If I limit my answer to just the slippers, then the answer depends on what's in bloom at the time, and size tends to be more of an attention grabber than color. So, for instance, the caudatum group and kovachii hybrids usually get some kind of reaction, the besseae and schlimii group rarely get a mention.
Overall, there seems to be an inverse correlation ... the more enthusiastic I am about a particular orchid, the less likely it is that non-plant or non-orchid folks will take any interest.
