Tipton & Coseley Building Society took second place, Hinckley & Rugby ranked third while Interbay and Furness Building Society took the forth and fifth spots.
Based on a standard set of circumstances, these lenders offered the most generous buy-to-let loan amounts.
For top slicing, MBT found Barclays and Clydesdale Bank to be the best while Saffron Building Society and the Darlington produced strong results for expats.
For holiday lets, Ipswich Building Society and Principality Building Society were the strongest.
Tanya Toumadj (pictured), chief executive at Mortgage Broker Tools, said: “Affordability calculations are important for buy-to-let investors who want to maximise their leverage, so it’s really interesting that the top five lenders for affordability are arguably lesser-known lenders in this part of the market.
“This shows the importance of shopping around and carrying out thorough research. Every case is different and affordability results vary depending on the individual circumstances of a customer.”