Fanjita`s excellent Eboot Loader for v2.0 firmware PSPs is a great piece of software but for those who want ease of use then a v1.0 (No chance of getting this firmware version) and also a v1.5 are the best ones to get.
Now a lot of people would never touch a downgrader with a bargepole as the risk of bricking (killing your PSP) is there, there may be a small chance but thats not gonna please the 0.01% who bricked theirs.
Luckily theres still some shops worldwide who have still got v1.5s (Legit and not Claiming falsely) in stock, heres the rundown on who has em and for what price: