well he's maybe in the conversation but Messina and Obradovic have much more overall european achievements. With all Blatt has achieved he's not even close. Obradovic and Messina have 12 Euroleague trophies together. with all due respect.
Messina to me is a little over-rated, sure the guy have 4 euroleague titles but he got most of them as a favorite. those Bologne teams had great rosters (including one young guy named Emanuel Ginóbili) and he got them a very long time ago (1998 and 2001) so I think there is a slight discount in their importance as time goes by. and those CSKA teams he won with were even more loaded with talent.
Of course Messina gets a lot of credit for actually winning but a lot of his teams should have won it anyway. look at Pablo Laso- this guy is a TERRIBLE coach, and he just won a euroleague title- his roster is just that good (you can actually make the case for their roster being better than some NBA lottery teams).
Sinca 2008, Messina haven't won a title despite getting a pretty much blank-check from Real Madrid (2009-2011), was part of the failing Lakers (2011-2012) and didn't manage to win again with CSKA despite, again, getting pretty much a blank check 2012-2014 (including getting completely out-coached by a guy named Blatt while having a guy deep in his bench with the same salary as Maccabi's starting five put together).
Obradovic is different, he has 8(!) title with 3 different clubs and a lot of this title didn't came with high-budget teams. but Obradovic is really not suited to the NBA in term of how he treats players and his behaviour:
View: https://www.youtube.com/watch?v=H_ABVXhAH1Q
View: https://www.youtube.com/watch?v=z-gyjtOnwA8
Sure Blatt have one euroleague title as head-coach but that title was with a team no-one expected to reach the final 4, he took mediocre Maccabi teams with very low salary budgets to compete at the highst levels, he had a lot of influence in the Maccabi team that went back-to-back Euroleague wins (2004-2005), and maybe the biggest success for him is with national teams:
He won Russia their first Eurobasket with a team built around AK47 and other guys unknown to people outside of europe against a Spain team with Pau and Marc Gasol, Jose Calderon, Rudy Fernandez and Euroleague legend Juan Carlos "la bomba" Navaro among others.
In 2011 he won a bronze medal in the EuroBasket losing to France (with Tony Parker, Noah, Batum).
And of course his 2012 medal in the Olympic games.
Personally I think Blatt is a better coach than Messina and not at Obradovic level yet, but I don't think his resume is any worse when you consider what he did- and who he did it with, in context I think his coaching resume is just as good as Messina's.