Not surprising, but kind of an easy vote for me.  Rush are in a tier all by themselves for me followed by Yes in the next tier by themselves.  From this group of 4 ELP would be next with Jethro Tull further down (to be honest mostly for their hits rather than their prog output) and lastly would be Zappa, who barely registers on my radar.  I have one of his first albums, which really didn't interest me all that much, and a "best of" album from him, but other than Valley Girl and Yellow Snow, I really don't know all that much from him.  With Jethro Tull and Frank Zappa, I never really considered them to be prog musicians until I started hanging out on PA.  This is more of a knock on my familiarity with them, than a knock on them.

With regard to Rush, having grown up in the 70s, I can tell you that initially no one thought of Rush as a prog band. They were a hard rock band. And their first few albums justify that label. There is nothing prog about them.

Rush then did become more progressive with 2112, Hemispheres, Farewell to Kings. I never thought of them as a group that totally became a progressive rock group, as they maintained a hard rock core, but the early prog influences of the BIG 5 certainly influenced Rush in the late 70s. As they entered the 80s they retained those prog influences, especially on stuff like Natural Science from Permanent Waves, but as the 80s wore on they became more of an AOR band with some prog influences, and moved in the same direction as Kansas. By the mid 80s they weren't really doing progressive rock as we knew it in the early 70s of course. So I have always been on the fence about thier progginess. They really are a band that really only had a 6-7 year period of prog in the late 70s early 80s. They certainly have been a gateway for many who had been exclusively into "hard rock" to get into prog, due to their arena friendly radio oriented style in the 80s where they won over huge number of fans. So you have to give them credit for attracting fans to prog, even though they were never fully a prog band like the big boys.

I would consider Renaissance, Gentle Giant, Hawkwind etc much more in the second tier of bands after the Big 5 before Rush, but there is no doubt they were an entryway into prog for many.

OK folks, let's put some historical perspective here, considering the four artists presented, before this thread is put under. 

Zappa's albums run the gamut of music.  You will find rock, prog, fusion, and any number of other genres (some previously unknown) in his catalog.  But he is not prog in the traditional sense (i.e., not typically grouped with ELP, Yes, KC, and whomever else you care to mention).

ELP is considered traditional prog, since they were one of the first bands working in what we would generally agree to be prog, along with Yes and KC and some other bands, yes even the Moody Blues.

Jethro Tull, while not perhaps initially considered prog, certainly dabbled in it on their early albums and became full-fledged members once TAAB was released.  I would consider this Tull's first prog album.  What happened afterwards is for another thread.

Rush is more difficult.  I hated the first Rush songs I heard, back around 1979.  You can't convince me that Working Man is prog, nor Fly By Night, nor any number of their early songs.  They were decidedly not prog in any then accepted sense.  I know this ecause the local FM-ROCK station had them in steady rotation, and believe me NO rock station was putting prog in the rotation at the time. As the band grew in skills, both songwriting and instrumentally, I could see how they would become to be conceived as prog.  For me this happened with Moving Pictures, though again this would have been more in the vein of a Zep type of transformation, where Zep moved from essentially being a really loud blues-based band (first album) to a band taking greater risks musically (say Houses of the Holy).  (Should add that Zep was also in steady rotation on said FM-ROCK station.)  Rush had that kind of progression.  So that is why I would not consider them a prog band in the traditional sense, no more than I would consider Led Zep a prog band.  Now Rush has had the good fortune of having considerably more longevity than Zep, so they have become viewed as a prog band.  And looking back, why not?  They have managed to run that gauntlet, to the point that I consider them, if not prog, then certainly 'intelligent' rock, or some variation thereof.  Which is what I think all of us who stumble onto this site are interested in.

Which, if you're still reading, was at least my reason for voting for ELP.