That's a tough one as they are all great games in different genres. Just go with what you fancy, oh except for POP as mont said. Terrible, buggy awfully made games. How they got a collection is beyond me.
God of War V1 & 2 are truely epic and enjoyable games.
MGS are high up as my favourite games of all time.
Sly started off great but soon got tiresome and repetitive halfway through Sly 2 to Sly 3.
Tomb Raider was OK but I already own Underworld and played Legend (which I didn't think I had at the time)so its a pretty pointless collection really.
Ratchet & Clank are on my list when they come down in price a bit more.
It really depends what your into because all the HD collections are previously released games if you've played certain games during your childhood then thats the HD collection you should go for
I grew up playing Tomb Raider, Ratchet & Clank and Jak & Daxter, so i already have the Tomb Raider and Ratchet & Clank trilogy's i just need to get Jak & Daxter when i can afford to