It's really hard to say, as without Nemesis, would B&M have become the go-to manufacturer for top class coasters in the UK? Without Nemesis, the big UK parks might be full of rides from Mack, Vekoma or who knows what?
I think you'd also have to split into into what enthusiasts thought was the best coaster and what the general public thought.
For enthusiasts it would probably have gone something like this, assuming Nemesis Inferno never got built either:
1994 - The Big One (or maybe Shockwave, but at the time I think most would've chosen The Big One)
1996 - Megafobia
1998 - Oblivion
2002 - Colossus (or would Odyssey have had a shot, given the lack of inverts in the country?)
2005 - Rita (mainly due to being something different at the time)
2006 - Stealth
And that's probably where it would've stayed.
For the GP you can really only look at the three biggest UK parks and stuff that either goes high, fast or has lots of inversions, so it would probably have been:
1994 - The Big One
1998 - Oblivion
2002 - Colossus
2006 - Stealth
2013 - The Smiler
The other big question I wonder about is whether Nemesis will still be the best after Blackpool's Construction 2018 is finished? For the good of the industry in this country I hope not, but it will probably always be my favourite UK coaster for nostalgic reasons.