hmm..
I can't do just 5.. but I'll add some logic / reasoning to my suggestions
i) Everything made by Bullfrog
Because it'd be criminal to make me choose between theme hospita/park and dungeon keeper, and lets face it, all their games are good
ii) Road Rash series (sega)
As much as i deteste ea they had something unique, not really anything out there like it
iii) Unreal Tournament (original only)
Alright, not an old game as such, but hear me out
Use a better engine, bring the cheat prevention up to scratch.
Add some new maps to existing but leave everything else alone
Every other fps is a frigging war simulator the people need a doom/UT/quake in their lives
iv) Final Fantasy series (PC)
Not to be confused with the mmorpg, I'm talking about the original games
The ports made to PC did not do the series justice because many people ended up simply ripping the playstation versions to PC because their game would not run properly. Biggest culprits 7 + 8
v) Strike Series (sega/psx)
Redo the whole lot ground up, maybe even roll all the existing game' storyline into a single game (cause these days the missions would seem short)