We can discount two of those because they were part of the band before they recorded anything. Sure Paul Di'Anno may have been their first recorded vocalist and the less said about the Blaze era the better. Realistically Bruce is the only voice of Maiden!

I agree. They were good with Ronnie, but nothing stayed too stable with Ritchie in those days.

Hawkwind can top that:

Current members

Dave Brock – vocals, guitars, keyboards, synthesisers (1969–present)
Tim Blake – keyboards, vocals (1979–1980, 2000, 2002, 2007–present)
Richard Chadwick – drums, vocals (1988–present)
Mr Dibs – vocals, cello, bass (2007–present)
Niall Hone – bass, guitars, sampling, synthesis and keyboards (2008–present)
Dead Fred – keyboards, violin, vocals (1983-1984, 2012–present)

Former members

Nik Turner – saxophone, flute, vocals (1969-1976, 1982-1984)
Dik Mik – keyboards, synthesisers (1969-1973)
Terry Ollis – drums (1969-1972)
John Harrison – bass (1969-1970; died 2012)
Mick Slattery – guitars (1969)
Huw Lloyd-Langton – guitars (1969-1971, 1979-1988; guest - 2002-2005; died 2012)
Thomas Crimble – bass (1970-1971)
Del Dettmar – keyboards, synthesisers (1971-1974)
Dave Anderson – bass (1971-1972)
Simon King – drums (1972-1979, 1979-1980)
Ian "Lemmy" Kilmister – bass (1972-1975)
Robert Calvert – vocals (1972-1973, 1975-1979; died 1988)
Simon House – keyboards, synthesisers, violin (1973-1978, 1989-1991; guest - 2000-2002)
Alan Powell – drums (1974-1976)
Paul Rudolph – bass (1975-1976)
Adrian Shaw – bass (1976-1978)
Harvey Bainbridge – bass, keyboards, synthesisers (1978-1991)
Martin Griffin – drums (1978-1979, 1981-1983)
Paul Hayles – keyboards, synthesisers (1978)
Steve Swindells – keyboards, synthesisers (1978-1979)
Ginger Baker – drums (1980-1981)
Keith Hale – keyboards, synthesisers (1980-1981)
Andy Anderson – drums (1983)
Robert Heaton - drums (1983; died 2004)
Rik Martinez - drums (1983)
Clive Deamer – drums (1983-1985)
Alan Davey – bass (1984-1996, 2000-2007)
Danny Thompson Jr. – drums (1985-1988)
Bridget Wishart – vocals (1990-1991)
Ron Tree – vocals, bass (1995-2001)
Jerry Richards – guitars (1996-2001)
Jason Stuart – keyboards, synthesisers (2005-2008; died 2008)

Former guest musicians

Michael Moorcock – vocals (1975, 1981)
Captain Rizz – vocals (1997-2000)
Jez Huggett – saxophone, flute (2000-2002)
Arthur Brown – vocals (2001-2003)
Jon Sevink – violin (2009)

The "classic" lineup would be the one with Lemmy & Bob Calvert

Red Hot Chili Peppers only found a decent replacement guitarist for Hillel Slovak in John Frusciante.
When he left the first time and currently, they have become bland.
Nightwish are not quite the same without their original singer.

For me the Space Ritual line-up was the best.

I can't believe the list didn't include Stacia Blake!!

She certainly had a classic line up.

It depends on the band really - one of my favourite bands, Fairport Convention, has had an ever-changing line up over the 46 years of their existence. Simon Nicol is the only founding member still in the band and he had a few years off in the 70s. Dave Pegg has been a member since 1970. They've had female vocalists and male vocalists, yet always retain a distinctive Fairport style. Other bands struggle after losing one member - The Undertones without Feargal Sharkey for example.
The line up change that made the biggest difference in my opinion is Genesis - I love the early Genesis albums, but when Peter Gabriel left they made a couple of half decent albums without him, then the loss of Steve Hackett turned them into a horrible pop band.

I think for rock bands it doesn't make as huge an impact when the guitarist, drummer, bassist etc leaves, because that sound, with the right skills, can be replicated.

In pop groups however, where all the members are vocalists, it's a little harder. It totally changes the sound of the group. Sometimes it works, sometimes it doesn't. For Atomic Kitten, it worked (sorry Kerry), for Hearsay, it didn't work. For Sugababes, it definitely worked the first time, I get lost after that.