I agree about Covert Action , and excellent old game with lot of replayability and still fun though i think the same the car stalking could have been made a bit better, and more relevant to your investigation as you don't even need to do it, the other choices will lead into the same discoveries anyways.
Hidden Agenda is another old jewel full of replayability and character, the only annoyance i had with it was when you took a decision, then you were interrupted by someone completely opposite to it and you had no other choice than to agree with him and so reject your original decision.
Sure you're not supposed to be a tyrant but a benevolent president following up a revolution that got rid of a dictator, but those nasty interrupt closed some doors on what you could do.
To add on the oldies list i enjoy and made
here and
there some other oldies i still play on my dosbox :
Sword of the Samurai, on the same concept as Covert Action (in term of the game made of mini-games) , very replayable , the weak point of it is the sword duelling, not that it's hard as once you get how it works it's easy, but that really could have been much better in gameplay than what it was. Though it's still better than the duelling in Pirates! but i don't think there's worse anyways than the sword fight in Pirates!
World of Xeen , combining Might&Magic 4 and 5 into a giant game , it's really good . The gameplay is still the same classic one as the M&M serie that came before it, create a party of various kind of adventurer and explore the world turn based, get into dungeons , find items, level up etc... You can get it on GoG nowadays.
Ironseed , a space exploration game i recently discovered (and that is freeware, bay12 thread
here, be sure to get the patch on the main website) , really need to read the manual to get some of the concept, but it's very interesting.
While adding links to mobygames entries, i can't help but find the new redesign really awfull, screenshots that takes time to load through some javascript , colors that strain the eyes, ... i wonder who they hired to make a redesign that bad and break what didn't needed repairs