Legion raid bosses by the order their IDs were added to the game.

[Deleted]

Ursoc: 				1841
Krosus: 			1842
Skorpyron: 			1849
Nythendra: 			1853
Dragons of Nightmare: 		1854
Tichondrius: 			1862
Star Augur Etraeus:		1863
Xavius:				1864
Chronomacic Anomaly: 		1865
Gul'dan: 			1866
Trilliax:			1867
Spellblade Aluriel:		1871
Grand Magistrix Elisande:	1872
ll'gynoth, Heart of Corruption:	1873
Elerethe Renferal:  		1876
Cenarius:			1877
High Botanist Tel'arn:		1886

No much can be derived from this but some simple bosses tend to be at the top, some are added in chunks, others not on the list are added between them (5man bosses potentially) and the bosses are mixed in ordering between the raids which might indicate they may pick and choose after the raid is made from bosses already existing without a home.

[Saiona]

Ursoc: 				1841
Krosus: 			1842
Skorpyron: 			1849
Nythendra: 			1853
Dragons of Nightmare: 		1854
Tichondrius: 			1862
Star Augur Etraeus:		1863
Xavius:				1864
Chronomacic Anomaly: 		1865
Gul'dan: 			1866
Trilliax:			1867
Spellblade Aluriel:		1871
Grand Magistrix Elisande:	1872
ll'gynoth, Heart of Corruption:	1873
Elerethe Renferal:  		1876
Cenarius:			1877
High Botanist Tel'arn:		1886

No much can be derived from this but some simple bosses tend to be at the top, some are added in chunks, others not on the list are added between them (5man bosses potentially) and the bosses are mixed in ordering between the raids which might indicate they may pick and choose after the raid is made from bosses already existing without a home.

Heart isn't very complex though it seems pretty easy. Mythic Tel'arn is going to be aids though lol

[Deleted]

This is probably the order of mechanical difficulty rather than difficulty of boss, Ursoc being single target and very few abilities up to tel'arn (lol)

[Deleted]

Regarding complexity some 5man bosses are more complex than a few raid bosses on mythic mode. The 5man situation is a bit odd. I went on a parsing of all of them and it appears certain instances are very simplistic in mechanics while others could be easily raid bosses on mythic mode. It appears that the bosses of 5mans and raids are often picked and chosen from a common pool of available bosses with the exception of a few iconic lore figures that must be in an instance or an instance's particular spot.

[Deleted]

Mannoroth: 1795
Hellfire High Council: 1798
Archimonde: 1799
Xhul'horac: 1800

It has absolutely no other meaning than to see the order of implementation.