[quote name='io']
Actually, I have played a few minutes of a few of these games on one of the Sonic collection discs (for the GC I think) and I found them to be frustrating and boring, for the most part. So maybe I shouldn't bother

. Though my kids might want to play them and I have tons of MS points so that's why I'm asking for the 2 "best", I guess.[/QUOTE]
The XBLA versions aren't too hard at all
if you remember to take advantage of the save feature. Just save every level or so (or mid-level if you like) and reload your save if you screw up.
I recommend Sonic 2 as it's the most fun part of the series and 2-player. Then you could pick either Sonic & Knuckles, which is its own game and also opens up new areas in Sonic 2, or just get Sonic 3, which is a good game but not quite as fun IMO as Sonic 2.
[quote name='theironunkind']is there any reason to buy these sonic games if i already have the ultimate collection?
[/QUOTE]
The XBLA versions of Sonic 2 & 3 have online play. More importantly, the lock-on feature of Sonic & Knuckles works with Sonic 2 & 3, making Knuckles playable and opening up new areas in those games. That feature is absent from the disc version.