A very inspiring thread, thanks again, gents.
Currently my most-played games are:
Gun Nac - Good old Compile, didn't realize that the game was made by them at first but it gave me a nice Aleste vibe, then I noticed the Compile logo. Brought a smile to my face. Grrrrrreat shmup with a nice difficulty curve and a more than generous amount of powerups that get thrown your way.
Contra and Super C - Well, duh. What can I say, fucking amazing and still a blast to play. Some of the newer versions and adaptions of the original concept are nice and all but they're nothing like the real thing.
Gun Smoke - Another arcade fave of mine, excellent port. The button setup works very well. Mowing enemies down with the machine gun feels great.
Batman (Sunsoft) - Wow, what a game. Kicks yer ass left and right but gameplay is absolutely spot-on.
Guerilla War - Nonstop action, just what I need. It's amazing how vast the game is, loads of levels, tons of enemies and installations to wreck, love using the tank.
Rush'n Attack - The C64 port of the game (called Green Beret) was my #1 choice for a long while (together with the great Speccy port, go figure). The NES port is superior because it's way more playable. It's considered a difficult game but I find the C64 version wayyy more difficult.
Mega Man, Castlevania and Ninja Gaiden - The big three, going to tackle the games one after another which will probably take me quite a while. The first Mega Man and Castlevania are almost finished but Ninja Gaiden is a tough nut to crack. Can't blame anyone but me for it, tho, gameplay is tops and immensely fun.
Spelunker - Played it a lot on MSX, the NES version is great because it's a bit more forgiving in terms of jumps.
Ghosts'n Goblins - I've been a fan of the game from day one ever since it appeared in a pub I used to frequent back in the day. The home port to play at that time was the C64 version and its developers did a stellar job considering the hardware limitations but of course it was lacking in many ways so I hoped it would come out on Genny that I bought later on. Too bad it didn't, although Ghouls'n Ghosts was excellent as well. I consider the NES version the best of the 8bit era, there were a lot of really horrible GnG ports on various home systems at that time so I never got why some people heaped so much critizism on it. The scrolling could be smoother and the difficulty is just downright evil but it's all there, plays very well and music and sound effects are just great. I'll beat it one day.
1942 - And another arcade fav of mine, the NES port is really good IMO.
Shatterhand - So much fun to play, looks and sounds great as well. Would love to see how this could have worked out on Master System.
There's more to try out for me thanks to your suggestions so the list is only a temporary one, may look different next month.