#3 Jake Diekman
For the last couple of years, Jake Diekman has been one of the more reliable left handers out of plenty of bullpens. The funky southpaw went 3-3 with a 3.86 ERA in 67 outings this season. What's really intriguing about Diekman is his ability to put hitters away. He struck out 12.3 batters per nine this season and has an 11.5 K/9 in his career.
The Mets had Aaron Loup to rely on to get the lefties out. With Loup leaving in free agency and Brad Hand a current free agent, the Mets don't have a lefty in their bullpen.
While Diekman is definitely tough on lefties, he can also be relied upon to get right handers out. Righties slashed .200/.286/.421 against him. He did allow 9 home runs which is high but Diekman for the most part was very effective against both hands.
Diekman's biggest weakness has always been his ability to throw strikes consistently. He's walked 5.0 batters per nine in his career. However, the walks aren't a deal breaker because he doesn't allow many hits, usually does not allow many home runs, (0.7 HR/9 in his career), and he strikes out a ton of batters.
The Mets have been great at finding high leverage left handed relievers the last couple of years with guys like Jerry Blevins, Justin Wilson, and Aaron Loup highlighting that category, I wouldn't be surprised if Diekman is the next arm to join them.