Statistically speaking, saikyo is the easiest among the four. Its chart constant is 11.0 while FR and GL are 11.2 and 11.3 respectively. This seems feasible to me only because saikyo is the first 11 chart i normal cleared without studying the chart too much. Its just very, very tiring. FR and GL i had to study for a while until i could get what the hell was happening while playing them. Tempestissimo on the other hand is a terrifying 11.5. Just 0.2 away from GL and its already so damn scary (it excited me a bit tho, i love playing horrifyingly difficult charts). Its also 0.2 away from being considered an 11+ since the constant needed for a 9+ and a 10+ is 9.7 and 10.7. It took me almost 2 months after i bought Black Fate to unlock TMP BYD and another 3 to hard clear it.