You'll have to define what you mean by "best". Is "best" a compromise between price and efficiency?
- If you're looking for a cheap solution, you might want to go for a magic swap thingy (+ possibly a replacement shell). This works pretty well except you have to patch some games to get them to work.
- If you dont mind the price, the HD Loader is cool. You get to boot games from a HDD. It's great, fast load times, doesn't damage your PS2 lens, but a few games aren't compatible. Oh well.
- If you have the opportunity, get it modded. In france I got it done for 80€. I gave the shop my PS2, they gave it back to me the day after and all was done perfectly. You can also mod it yourself, that'd be the best way!