I’d argue 10 years ago was close to the best crop of HW’s at the end of Pride era, even 5 years ago was ridiculously better than the current top 10...2015 was unfortunately completely stagnant because HW had Cain Velasquez holding up progress within the division for years on end.
Note the lack of UFC fighters ranked in the top 10 during the 2005/07 era, with the exception of Arlovski, Silva, and Couture the UFC was barren of talent at HW.
Apologies for the shitty format, bad copy and paste job
HW Rankings Jan 2005
1 Fedor Emelianenko 23-1-0
2 Antonio Rodrigo Nogueira 25-3-1
3 Mark Hunt 2-1-0
4 Mirko Filipovic 16-3-2
5 Andrei Arlovski 9-3-0
6 1 Josh Barnett 18-2-0
7 1 Sergei Kharitonov 13-3-0
8 4 Ron Waterman 13-3-2
9 Lyoto Machida 6-0-0
10 Tim Sylvia
HW Rankings Oct 2007 (when Pride was purchased by Zuffa)
1 Fedor Emelianenko 27-1-0
2 3 Randy Couture 16-8-0
3 -1 Antonio Rodrigo Nogueira 30-4-1
4 -1 Gabriel Gonzaga 8-2-0
5 1 Josh Barnett 23-5-0
6 35 Cheick Kongo 11-3-1
7 -3 Mirko Filipovic 22-6-2
8 -1 Mark Coleman 15-8-0
9 -1 Mark Hunt 5-3-0
10 -1 Tim Sylvia
HW Rankings Jan 2010
1 Fedor Emelianenko 31-1-0
2 Brock Lesnar 4-1-0
3 Antonio Rodrigo Nogueira 32-5-1
4 Frank Mir 13-4-0
5 Junior dos Santos 9-1-0
6 Josh Barnett 27-5-0
7 1 Brett Rogers 10-1-0
8 3 Cain Velasquez 7-0-0
9 1 Fabricio Werdum 13-4-1
10 -1 Shane Carwin 11-0-0
HW Rankings Jan 2015
1 Cain Velasquez 13-1-0
2 Fabricio Werdum 19-5-1
3 Junior dos Santos 17-3-0
4 Andrei Arlovski 23-10-0
5 1 Travis Browne 17-2-1
6 -1 Mark Hunt 10-9-1
7 Antonio Silva 18-6-1
8 Ben Rothwell 34-9-0
9 Alistair Overeem 38-14-0
10 Josh Barnett
Current HW Rankings
1 Stipe Miocic 19-3-0
2 Daniel Cormier 22-2-0
3 Francis Ngannou 14-3-0
4 Junior dos Santos 21-6-0
5 Derrick Lewis 22-7-0
6 2 Ryan Bader 27-5-0
7 -1 Curtis Blaydes 12-2-0
8 38 Jairzinho Rozenstruik 10-0-0
9 -2 Alistair Overeem 45-18-0
10 -1 Alexander Volkov