I can't even say who would be the most replaceable without hearing what the band would sound like without that member. I'd almost say Colin.
In case of PT it's just like with DT, if anyone from PT left, I'd be in an equal state of WTF + shock + "what happens now?"
nuggets I know like after MP left DT. I just can't imagine it. It's like the perfect construction falling apart. It's interesting to see what's the different way to reconstruct it, as there's no way to bring it back how it was, but still I wish it remained like it was. I don't want anyone leaving PT.
Steven - obvious. The core.
Colin - musically, his style is just unique. I love his playing and how he can actually lead the song with bass. Personality - come on, it's Colin. He's not the most conspicuous person on Earth, but PT without Colin and the permanent mysterious smile on stage? Come on, wtf.
Richard - if you're all about TEH RIFFZ MAN, you probably won't notice him. If you pay attention to the music, removing what he does would be literally stripping the music of... something extremely important. What he does is like laying foundations and the rest of the band builds upon it. The most obvious case of this is, ironically, the album that is often mistaken for the heaviest and most guitar-driven - FOABP. Listen carefully, and try to imagine this album without RB. 80% of the album's dark atmosphere is created by him basically. I needed time to realise it but really, the guitars are truly like the icing on the cake on this album. The rhythm section and Richard's sound effects are basically 90% of the album.
Gavin - my favourite drummer. MP is absolutely awesome but I still have to put him #2. Absolute master in combining complexity, subtlety, and finesse. I don't think he's ever played such thing as a simple drum pattern on a PT album, and it's never like he's drawing attention. It's amazing, but not over the top.
John - he's got a great voice to complement SW, and does a really great job live overall. I really wish he became a full-time band member.
So... there you go.