Cyber Stadium Series: Base Wars (Konami, 1991)
Chip & Dale's Rescue Rangers (Capcom, 1990)
Devil World (Nintendo, 1984 in Japan, 1987 in Europe)
Mega Man 1 and 6 (Capcom, 1987 and 1993)
Mega Man 1 and 6 are generally considered the weakest entries on the NES platform, but both have some interesting quirks of their own. Mega Man 1 was certainly among the earliest games that allowed the player to tackle stages (and bosses) in any order, though it did become somewhat notorious for its high difficulty level and lack of later series innovations like passwords and E-tanks. Mega Man 6, on the other hand, definitely felt a bit rushed as it came out in the twilight days of the system and had comparatively bland level designs and weapons compared to its predecessors. However, it did also have some unique power-up suits in the Rush Armor (which can smash through walls and deal damage with short-ranged punches) and Rush Jet (serving as a jetpack that allows flight for short distances). Regardless of their shortcomings, though, they're both a lot of fun to play, and as I've said before, classic Mega Man at his weakest is still better than most game franchises at their best.
Monster Party (Human Entertainment, 1989)
Fun fact: An unreleased Japanese prototype of the game reveals that many of the bosses within are actually parodies of famous films, from Planet of the Apes to Alien to Gremlins to The Thing. Unsurprisingly, most of these were changed in the US version because of copyright concerns.
Summer Carnival '92: Recca (Naxatsoft, 1992)
Zanac (Compile, 1987)
Created by the company that would later bring us such classics as Gun-Nac, Guardian Legend, MUSHA and the Aleste series, Zanac is the grand daddy of them all, and a damn impressive game in its own right. Not only did you have ten distinct weapons to choose from, but the game also sported a pretty solid, energetic soundtrack and blazing fast speed. The visuals are a tad on the ugly side, but when you have gameplay this solid and fast-paced, that really doesn't matter much, now does it?
Zoda's Revenge: Startropics II (Nintendo, 1994)
One of the very last games officially released for the NES, and it was quite a good one. A followup to Startropics - a game curiously programmed by a Japanese development team but only ever released in the west - Startropics 2 featured tighter controls, crisper graphics and a new plot involving Mike travelling through time searching for the seven magical Tetrads. Oh yes, Nintendo had the Tetris license at the time and they wouldn't let you forget it.
Another welcome change was the general softening of the difficulty curve - while still very challenging, cheap shots that took off a third of your health bar were now a thing of the past, and making contact with the bosses wasn't an instant death sentence - you simply lost a large chunk of your health. Just a touch more of the problem-solving aspect and solid narrative of the original and this may very well have surpassed it!