Rockman No Constancy - I have played that, it's probably the best MegaMan experience on the NES, the way the recreated the first level of MegaMan X is just awesome. Zelda Outlands is another nice romhack, so hard though. I did a few articles about those a while back. There's some killer Sonic hacks too, so professional!

Totally Rad is one I used to own when I was like 12...super good rock tunes on that one, insane bosses. Another weird one I happened on was Low G Man - imagine megaman or metroid, but you can jump about 4 screens high when fully powered up.

Thanks to all for your comments, I'm slowly working through the list. One I stumbled on that was pretty cool was SCAT Cybernetic warrior - kind of like contra/gradius. It has a pretty cool system that allows you to reposition your "options" in real time. Its WAY too hard though, I can't even pass level 1, it's the closest thing to bullet hell i've seen on the NES.

I tried Cocoran, no idea what was going on there!!

oh darn, I totally forgot to throw Totally Rad in my NES list !

..

How did I never play Shatterhand??  Controls are a little sticky, but its good. Also the first boss is totally M. Bison with a gun.

Sad thing is there is no save state working for NES, that limits any real progress to password games (i just don't have the whole day to play NES anymore)

They use a picture of Little Samson for my weeklytreats track in 2013 !
http://weeklytreats.bandcamp.com/track/thrashboy

Shatterhand is great, but STUPID hard! Gimmick! and Monster Party are also great, but less hard, at least for me.

Bio Miracle Bokutte Upa is quite the trip and has some interesting mechanics, the level design can get a bit archaic and the bosses are kind of repetitive, but it's still pretty fun!

There's also the megaman romhacks, particularly Rockman 4 Minus Infinity and Rockman No Constancy.

Uhhhh, I don't know how underrated Little Samson (and the AMAZING WeeklyTreats glitchy romhack that LTC did of it!) and Blaster Master are, but they're great as well.

NES:
Eliminator Boat Duel
Skate or Die 2
Cheetahmen II

dood, Zombie Nation is super wacky fest... I can only think of a couple other wackier nes games, from the action 52 cart:

MASH MAN

NON HUMAN

but they are pretty much completely unplayable, haha.

NES :
Getsufuu Maden
STARTROPICS !!!!!!
Moon Crystal
Paperboy
Sugoro Quest
Sweet home (Suito Homu)

SNES :
Treasure of the Rudras
Alcahest
Arcana
Cyber Knight
Dual Orb 2
Front Mission
Ogre Battle
Tactics Ogre

had to youtube samurai zombie nation, its pretty fucking mental, you're a floating zombie head thing.....

NES: Gunnac, mr Gimmick, Crisis force, Shatterhand, Metal Storm.
Image fight conversion is also nice, but has the shittest music ever. (great fun though)

if you wanna tear your hair out play "Takeshi no Chōsenjō "
"Samurai Zombie Nation" will blow your mind and is a must!
"chiller" is another MUST...

ohh and if you have access to famicom games get "Splatterhouse  Wanpaku Graffiti"

If you've never played Act Raiser before it's a personal all-time favorite. Mix between side scroll fighting/platforming and a top-view civilization builder.

yes metal storm!! i remember that one! the most insane boss battle ever!

I will definitely be playing some Metal Warriors for SNES too...with the best 2p vs ever.

Pocky and Rocky on SNES was pretty good/cute/difficult.
On NES, Little Samson, Crash and the boys street challenge, and Darkwing Duck were pretty underrated imho.