as someone who is slightly claustrophobic what is the best gas mask?

Save your money.

Gas masks only work if you have the complete suit on. In the event of a chem/bio attack you have about 9 seconds to get your mask on before you die.

Gas masks also wear out after about a decade or so. The seals will go bad. The filtration systems need to stay sealed in their original container. Once they are unsealed they are only good for about 6 months. In the event of an attack they go bad in about 30 minutes.

Actually, the one that WORKS when you "Don, clear and check" it.

After that, possibly one that gives you decent field of vision. I always liked the M-17A1 for it's firm fit and ease of maneuverability. But it does have two separate eye ports, and you may not like that as much as one that has a face shield. I am also blind as a bat, and the M-17's will accept a small spectacle insert with your prescription. So I am biased.

Good luck finding one.