Depends what people mean by underrated. If people are saying he (Cristiano) isn't top 30, then yeah he is being underrated. Anybody that says he's top 10 though is overrating him quite clearly, his resume pales laughably to the people that belong in that company.
The funny thing about his "rivalry" with Messi is that he is benefitting off hugely from being in Messi's shadow. People feel as though because he's no 2 behind Messi (I don't agree with this but that's another story*) he is "unlucky" and in many other era's he'll be the best player in the world.
This is also analogous to the Current Barca-Madrid rivalry, where I feel they are looked at higher in Europe because they are the no 2 team behind Barca in La liga (wrongly might I add), again though, that's another story.
Right, so let's stay on topic, what era(s) would Ronaldo be the best? I'll start from 1970
1970-1975? Well a superior version of Ronaldo, Gerd Muller was behind Cruyff and Beckenbauer in that era so no.
1976-1980? Perhaps, this was the Kevin keegan era so yes.
1981-1985? Platini era, Platini was a 1 in 2 goals player while also being one the greatest playmakers in European football, so nope!
1986-1990? Maradona's era, again nope.
1990-1995? Another relatively weak era with George weah, Maldini, Roberto Baggio, Laudrup, Stoichkov being the standouts. But with Maldini being in his prime here, would you honestly say Cristiano is clearly better in his position than Maldini was in his position? this is Maldini; one of the greatest defenders in history(in two positions mind you).
1996-2000? Ronaldo and Zidane era, no chance.
2000-2005? Henry era, ok he could take this one.
2006-2010? Ronaldinho at his peak was better in the earlier part of this era, Messi was better from 2008 onwards. He deserved his award in 2007 though, arguably in 2006 too.
2010-Present? The gap between him and Messi have widened and will probably continue to get wider.
Going back to 1970, he would probably be the best during 1976-1980 era and arguably the 1990-1995 era. Thats 2 out of 9 era's, the other 7 had better players. And I didn't even go back to the Di stefano, Eusebio, Pele, Garrinchia era. Where he also would be in the lower tier.
Basically, he should be thankful for being in Messi's shadow, because ironically,that is shielding him, in the sense that:
Messi is an all time great and Ronaldo is no 2 >>>> therefore Ronaldo is high up there on the all time list in the eyes of a consierable no of people.
*
I think Xavi is deserving of the accolade for being a better player than Ronaldo over the past 5 years. He is the common denominator between Spain 2008-2012 and Barca 2008-2012; two of the greatest sides in football. Not even Messi has that.
His 08/09 season is arguably the best of any CENTRAL midfielder in the history of the game . 30+ assists while absolutely dominating teams in every spectrum of quality; from tenerife to Manchester united, whether its with Messi (Barca), or without Messi(Spain).
His big game performances eclipses that of Ronaldo and he has also won a shit ton of more trophies than Ronaldo has in that period. I don't even think it's a controversial opinion with people that aren't casual football fans.