So, can we definitively say that MTM is the best place to go for slim fitting shirts? Otherwise, find a shirt off the rack and have it altered?
Yes. That said, if you go with an online maker, it's imperative that you know what your true measurements are, and in that regard, a well fitting otr option that has been darted is a good starting point. We've all seen countless examples of Modern Tailor trainwreckes.
Do my BB's extra slims fit like my mtm shirts? No. But do they fit better than 99% of what you see men wearing? Yes.
Frankly, some of my mtm shirts could be darted because I have a shallow lower back compared to my upper back. Alot of the really nice fitting stuff you see on here is a combination of great mtm and a body type that lends itself well to the particular shirt being worn.