Bit of a strange time frame.
I think Ferguson, del Bosque and Mourinho would definitely be the front runners, with Benitez and Ancelotti just behind. A lot of people write off Mourinho because of the resources he has had at his disposal at Chelsea and then at Inter, but his record simply can't be ignored, it's pretty phenomonal. 7 league titles in 3 different countries, 2 European Cup's with 2 different clubs, a UEFA Cup, unbeaten at home in 8 years or something ridiculous like that (with 3 different clubs in 3 different countries) and numerous other trophies too. All of that in the space of 9 years, and to counter the argument that he can only have success with an open cheque book, you only need to look at what he did with Porto; winning the UEFA Cup and then the Champions League in successive seasons with a team who have a relatively modest budget in comparison to the likes of Manchester United and Real Madrid. It'd be pretty petty not to put him near the very top at least.
It'd also be petty not to put Ferguson up there. Four league titles and a Champions League (plus a runners-up spot) says that you do. To say that Ronaldo won everything for them is a little silly too, especially as he helped develop him into the player he became after signing from Sporting to replace Beckham.
Of the rest, del Bosque has to be in there for winning both a European Cup and a World Cup (as well as a La Liga title), whilst Benitez (we all know his achievements) and Ancelotti (1 Serie A, 1 Premiership and 2 European Cups) are both in there too. There is an argument to say that Benitez is the best to be fair; he managed a Valencia side that didn't have anywhere near the same resources as Barca and Real Madrid (and inherited a side that had lost a lot of key players following a season in which they reached the Champions League final) to 2 La Liga's and a UEFA Cup, whilst we know what he did in 2005 with what was a largely modest set of Liverpool players.
It becomes more difficult after those five though. Some have said Guardiola, but it's difficult to know just how much managerial acumen actually goes into managing a Barcelona side possessing so much talent in the squad. He did play his part in unifying the squad once more and rejuvinating the side, but the side he inherited was largely built by Rijkaard, who took over at a difficult period for Barca and moulded them into the finest footballing team in Europe, with the help of Ronaldinho, Eto'o and co. So for that reason, I'd put Rijkaard ahead of Pep.
Wenger needs to be included too I think, despite his recent trophy drought. He produced the finest attacking side this country has seen since the great Liverpool team of the late 80s and won two titles along the way, as well as reaching a CL final. He's also working on a relatively modest budget too and his been for the best part of the decade, so he commands a place in the top ten.
For the final three places, I've gone with Louis van Gaal, Marcello Lippi and Fabio Capello. van Gaal has won titles in Germany and the Netherlands and has done so playing good football, whilst Lippi's success with Italy in 2006 puts him in there, and Capello who won titles in Italy in Spain. There were a few other candidates but there are too many question marks hanging over Mancini, whose record at Inter would ordinarily put him in there, but the match fixing scandal means his titles with Inter have to be weighted against the circumstances. I mentioned Pep before and why I haven't included him, but there's also Paul le Guen, who won successive titles with Lyon but failed with Rangers; Manuel Pellegrini, who produced some brilliant footballing sides during the decade but didn't manage to win a trophy of note; Scolari, who failed at Chelsea despite his achievements with Brazil and Portugal; Hiddink and Eriksson, who despite his failings did manage to get the very best out of the England team for a good few years. Honourable mentions to Renhagel, Hitzfeld and Didier Deschamps too.
1. Mourinho
2. Ferguson
3. Benitez
4. del Bosque
5. Ancelotti
6. Rijkaard
7. Wenger
8. van Gaal
9. Capello
10. Lippi
