It's no secret that the best center on the St. Louis Blues is Robert Thomas: 81 points in 70 games, and then an additional eight points in seven games in the playoffs. There is no question that he will be the Blues' top-line center as long as he's in the lineup.
Who will center the second, third, and fourth lines, however, is a wide open field.
Last year, captain Brayden Schenn was the second-line center by default--there wasn't really anyone else who could handle that sort of role. The problem, though, is that Schenn--in this stage of his career--may not be suited to second-line minutes and responsibilities. Yes, 50 points in 82 games is fine, but 5-on-5 47.54 CF% and 47.14 xGF%, per Natural Stat Trick, isn't going to cut it; the Blues were losing the possession and scoring chance matchup whenever Schenn was on the ice.
Adding Pius Suter as a free agent, however, can push Schenn into a more appropriate role. Suter was tasked with top-six--and eventually top-line--minutes with the Vancouver Canucks last season after JT Miller was traded to the New York Rangers and Elias Pettersson went down with an injury. There's versatility to Suter's game, and he can play any forward position, but locking him in as a rock-solid second-line center is the perfect role for him. That would make Schenn the third-line center, where he should be able to regularly outdo the competition.
Lastly, there's new acquisition Nick Bjugstad, who, like Suter, can play any forward position. It seems likely that Oskar Sundqvist will be the fourth-line center, with Bjugstad playing on the third-line wing--but there's a world where Bjugstad takes over 4C duties, instead, and the Blues would be able to run with formidable center depth. Expect a lot of different combinations and some trial-and-error in the preseason and early in the regular season.
GM Doug Armstrong did well in the offseason to shore up a position of weakness on the Blues' roster. Not only did he get proven NHL centers, but Bjugstad and Suter can both play any role in the lineup. That sort of versatility and flexibility will serve the Blues well in the regular season and, hopefully, on a playoff push in the spring.