There are a couple ways you can look at this: what has gone up the most, or what was the best deal considering prices at the time.
If it’s the former, anything bought a few years ago will be good, as mentioned above. Realistically we’re all just waiting for anyone who bought an Illustrator for less than 10k to chime in 
For example, there’s an old thread here (unfortunately I can’t find it atm) where a member won one for about $6k IIRC. I’m tempted to say this became the PSA 10 copy but there are so many strings to follow there I really don’t know. Absolutely do not take this to be anything other than wild speculation/pie in the sky.
Not mine, but here is a thread where someone won a 2002 No. 1 Trainer for 72k yen. At this point I’m just plugging old posts… I will always encourage people to go back and read them!
For me/single items would be either a Mew Victory Orb that I bought for 50k yen (about £400 at the time), or a 1ED Torchic gold star for about 9k yen (about £70), both of which graded 10. I probably meant to sell both of them years ago, but was too lazy to do so. Of course, everything I did sell I should have kept as well.
Further proof that fat hands > diamond hands 