This is a pretty interesting case. The vic talked about suicide, but unless he really, really, really wanted revenge against the ramen shop, there was no reason for him to commit suicide to run it into the ground. It also helps that we have three suspects making it less likely its a "how-dunnit". I wonder if he was planning to fake being poisoned/ use less than lethal drugs (like an emetic) to drive the shop under in order to acquire it because his own business wasn't powerful enough through finance. Maybe he cooperated with the real culprit who backstabbed him by using real poison?
I am looking forward to hearing about where the poison was found. The ramen shop people didn't seem to know that Saizu Tokumori was coming which limits their ability to prepare something ahead of time. It would also be surprising that they had poison on them ready to go. (Unless they were waiting for the guy to show up one day.) I suspect Saizu knew something about the dead regular because he went straight to that spot and knocked over the flower cup. Saizu might have also been involved with the guy's death.
In general, I don't think the poison was put on the chopsticks on the far right even though no one should sit there. That's because someone sitting in the second seat from the right might reach for them, even over the flower vase. This creates the possibility of poisoning an unintended target. There were no chopsticks in the bucket in front of Conan's seat so the option was to go for the far left ones or the far right ones. Conan went for the far right.
Of the suspects:
Ogura Katsumasa could have put the poison in the ramen. I am going to rule that out because that would be too straightforward to make a case. He was behind the counter most of the time, so he didn't have many opportunities unless he was going to toss pellets of poison. Poisoning someone in his own shop would be bad for business and he seems to have the will to want to keep the shop alive, even if he hated Saizu. It would be better for him to call Saizu out somewhere to talk about selling the shop and then poison him there.
Oohashi Sayo could have put the poison on the bottom of the glass she slammed down, poisoning the puddle and anticipating it would soak through the handkerchief and get on the Saizu's fingers making it look like a suicide. She could have put it in Saizu's waterglass as well, but again too straightforward.
Taninaka Atsushi, the barber, has the best tricky strategies and most likely the advantage of knowing Saizu would go the ramen shop after getting his hair cut. He prevented Conan from having chopsticks passed from Saizu, which could indicate he's trying to stop Conan from being poisoned. (It reminds me of the poisoning case right before Golden apple case.) He could have pickpocketed Saizu's handkerchief or smeared poison on his clothing in a specific spot while cutting the guy's hair, meaning the vic walked in with the poison on him. Also poisoning Saizu in the ramen shop might put it out of business and allow him to acquire the land to expand the barber shop's footprint.