Interestingly, the aircraft with the best economics is actually the B757 but it's just a bit too big for the low cost market.
, EZY used 737s which were leased from IAG using GB Airways' AOC because they were following Southwest's business plan. And they started with two aircraft, not one!
Ryanair was also copying Southwest, so they too used 732s initially - acquired from Britannia, as I recall. Prior to that, they had used BAC1-11s.