Easy, PS2 by a landslide.

Is every PS3 game also available digitally?

PS1 and PS2 emulation is easy on PC, so PS3.

PS2 No question! The RPGs! That was gaming before 1st Person Shooters became the default AAA genre.

I know Kingdom Hearts 1.5 and 2.5 aren't at least.

I'd love to say PS1, because I've been meaning to replay FFVIII for a long time, but while I own a copy, I don't have any means of playing it. Regardless, I choose PS3, because I have plenty of Ps3 games but no console to play them either xD

Well given my PS3 does an excellent job with the PS1, I'd take PS2 for the PS4.

I chose PS1 because we know PS2 and PS3 will NEVER happen because Sony already sell those on PS4 (PS2 with trophies and PS3 with PSNow). But PS1 disc based emulation and/or digital games could still happen.

PS2. Tired of PS3 games.

Well, I think the best would be betwenn PS1 or PS2.It would be great if it had backwards compatibility with the PS3 from the start, but now I think it wont make much of a difference.Especially that its somewhat cheap to buy a PS3 now, and there is also PS Now.The best would be the emulation of PS2 and PS1 games.Not only there is a market for it, since there are alot of people(me included) that love playing the classics, but also that finding a working PS2/PS1 is either hard or expensive, not to mention that it would give a headache to find the systems and games for it.

Edit:This post was done more with the thinking that you would be able to buy the games digitally.But a physical option would also be awesome, and facilitate alot for those that want to do so.