The most stable...? In my experience its been the silvers... They never get 'unstable.'
And as for the NTTs, I'd like to call them the 'discus' of the datnoid world lol. They require (almost) perfect water conditions, and even then its a gamble... There is a thread on this forum about a member from the UK trying to 'cure' the datnoid disease.... Just do a quick search and you will find the article.
I recently had an issue wherein I ordered a couple of NTTs online and the shipment was delayed by a day. One of the NTTs already had the Datnoid Disease tumors all over its body, it happened within a span of two days.