Depends on what you mean by "easy" As far as installation, fliptop and slide card are pretty easy.
On the other hand, both of those methods are a pain in the ass to use, as they require you to swap disks every time you boot up an imported/burned (FBI!) game.
I modded my PS2 with a DMS4 EZI no-solder mod chip. It was kinda a pain to install (taking apart a v5 PS2 so you can get to the motherboard is a pain in the ass!) But it's relatively easy once you get there. The clips snap on to the DSP and BIOS chips, and you just have to plus a few cords in here and there. Honestly, the only hard part is taking apart/putting back together the PS2. (watch out for those damn tiny fan-wires!)
Also they don't tell you that you need to burn a software update disk because due to legality issues they can't sell the chip with the software already on it, but after about a 2-3 hour installation and burning a CDR, you're all set, and you never have to worry about swapping disks or anything like that.
So in the end, I have really good functionality (I can even disable the PS2 logo screen so games boot up faster, and it works with damn near 100% of the games out there) But there were a ton of "Oh shit, did I just break my ps2?" moments leading up to there.
...seriously.