Well, it's kinda tough to make suggestions for "good but inexpensive", since people tend to hold on to their good games and therefor they won't be traded away that often, so they fetch higher prices.
Sonic 3 should be available in your intended price range. Should you find Comix Zone for under 10, I'd suggest getting it as well.
A personal favorite of mine that's rather rare but should be cheaper than 10 pound (at least I got it for 7 or so) is Second Samurai.
If you like Ecco the dolphin then you should also get Ecco II, if not then give it a pass.
If you like Soccer games get Sensible Soccer, I think that one's aged better than any of the FIFA-games.
Cool Spot is also a pretty decent game that should be available for cheap. Don't pick up "Spot goes to Hollywood" by mistake, though.
Shooter-wise Thunderforce III and IV are definitely worth getting.
And sorry if I came off too harsh, I only wanted to express how big the "best for the system/games for cheap" divergence can get (also, you wanted people to "Rate your MD collection", so... yeah). Still, If you could give some more specific preferences about what you like and what not, it would be easier to give you a few suggestions.
Also, Anyone who was wise enough to get Speedball II ranks high in my opinion!

(now don't you DARE say it's not good

).