Good? Yes The best? No Sonic CD is a good game and well worth checking out, but it's not the greatest
Right in this game Sonic has less of a focus on speed and more of a focus on platforming, which is fine. Levels are reasonably short and very easy. There are A LOT of enemies present, so stay on your toes. There are two acts in each world where you must simply get to the goal and then a boss. In this game the bosses don't make you jump around on Eggman. Each has a spacific gimmick to defeat it. One has you running along a conveyor belt dodging spike balls, the friction from the conveyor belt will harm Eggman, another will have you in a pinball type area where you will have to use flippers to flip your self up to the top of the area to defeat Eggman.
Now to mention Sonic CD's best feature... Time Travel! There are four time periods, present, past, good future and bad future. You always start out in the present (with the exception of boss battle which always take place in the future) You can travel through time when you pass a time post labelled either past or future. Then you must run at top speed for a certain amount of time and if you do this successfully you'll warp through time. The stage layout will change slightly and the enemies featured in the past are faster and more dangerous than the ones in the bad future which are slow and weak (the present time will have a mix of both normal and damaged enemies) The good futures can only be visited if you go back in time to the past and destroy some weird machine in each level... Wait... is this game trying to say something? If we don't do something to change the future we'll end up with a virtually destroyed and not very nice world?
Alternately to get good futures you can collect the seven time stones. How do you do that? You finish a level with 50 rings and jump into the bonus ring and then complete the special stage. In the special stage you will run around a 3D perspective environment and have to destroy all the UFOs within a time limit. this is not as easy as it sounds as you will frequently mis-time jumps (at least until your skills grow) and fall into water which will decrease you time limit.
Earlier I mentioned I didn't find the gameplay as fun as other Sonic games, I don't know why I just find the level routes slightly bland and control feels a bit too tight. I actually don't know why I didn't enjoy this as much as other Sonic games.
As for graphics, the actual level art is probably the best seen in any Sonic game, the colours are bright and vivid and the actual art of the levels really appeal to me. It's alot less realistic and natural than that of other Sonic games
Music: Interestingly the Japanese and US soundtracks are different. Over here in Europe the Mega CD version had the Japanese soundtrack, but the Gems Collection port uses the American soundtrack. The Japanese soundtrack has better and more funky tunes, whilst the US soundtrack is more atmospheric and adds an almost eerie atmosphere to the game play at times. check out Tidal Tempest bad future...
All in all. I enjoyed Sonic CD, whilst the gameplay isn't the best, the time travel feature had me coming back for more just to see how the levels look in different time periods. The level art is probably the best seen in any Sonic game This is the best game on Sonic Gems. Pick this up if you see it, it's certainly nothing you'll regret buying