While I do think you can compare the greatness of players from any time, it's probably more fair to make 'greatest' lists from certain eras.
IMO there have been 3 main eras in rugby: The 'old-school' era (pre 1980), the 'pre-modern' era (1980 - 2000) and the modern era (2000-)
For the 'old school' era, I think there are two clear standouts as the greatest: Gareth Edwards and Colin Meads. They were both incredible and always have to be right up there in any 'all time greatest' discussions. There are plenty of others too though... the likes of George Nepia, Frik du Preez, JPR Williams, Brian Lochore, Jack Kyle, Barry John, Willie John McBride, Mervyn Davies, Mike Gibson, the Pontypool front row, and many more, are all worthy of being in the discussion.
From the pre-modern era, there's Jonah, Serge Blanco, Michael Jones, David Campese, Jonh Eales, Danie Gerber, Jonathan Davies, Sean Fitzpatrick, John Kirwan, Philippe Sella, Hugo Porta, Jason Leonard, Christian Cullen, Keith Wood, Zinzan and Buck, Joost Van Der Westhuizen, Naas Botha etc. etc.
The 2 that stand out for me as the greatest from this era would be Jonah and Michael Jones, with Blanco, Cullen and Eales right behind them.
The modern era has produced many greats in a short amount of time. To list a few that standout in my mind... Richie McCaw, Dan Carter, Brian O'Driscoll, Tana Umaga, George Greegan, Paul O'Connell, John Smit, Lawrence Dallaglio, Os Du Randt, Augustin Pichot, Jason Leonard, Martin Johnson, Victor Matfield, Johnny Wilkinson, George Smith, Brodie Retallick, Sergio Parisse, Shane Williams, Jason Robertson, Brian Habana, Ma'a Nonu... the list goes on.
IMO the greatest of this bunch, and any era, is McCaw. Just so many factors that I can't be bothered explaining go into this man's greatness. IMO he is the clear GOAT. After Richie, I'd say Carter and BOD are the greatest of the modern era.
If I were to list my all time top 5 greatest players, it'd be:
1. Richie McCaw
2. Jonah Lomu
3. Gareth Edwards
4. Dan Carter
5. Colin Meads.
That may seem like blatant Kiwi bias, and maybe it is, but I honestly believe these guys have the best overall combination of performance and impact had in their times, greatest legacies formed, most and best achievements, and biggest mark left on the game.