Someone never played the 8-bit Contras or the arcade ones (which is to say, the good ones).
I have played every Contra except the 3d ones and the game boy ones, and I 1 life cleared Contra 1 way too easily. Imo Contra 3 is the best because it has the best balance between platforming and bosses. Contra 1 and Super C (both the Nes and Arcade ones) are good but it's a long shot saying they are the only "good Contra's".
IMO Contra 3 > Contra 4 > Shattered Soldier > Contra 1 > Hard Corps > Super C
Haven't tried to 1cc Hard Corps Uprising yet so I won't rank it for now.
For the most part, Metal Slug 1 and X don't stop your movement much to make you fight more waves (the big exceptions to this are stages 2 and 3 in MS1). MS3 does this constantly.
Still have no idea what you're talking about. 3 has waves, but it changes up what you're doing constantly. One minute you're moving forward, than fighting a wave, than in a vehichle, than swimming, than fighting a boss, etc. The key to a feeling of variety is that no two segments require the same strategy, and MS3 accomplishes that.
Also "pacing" should in no way be hampered by fighting. Beat em ups, 3d character action games, shmups, and numerous other genres do not force you to move forward constantly across every section, and they're "pace" isn't hampered by it in the least. Though that's because (with almost all good action games), what you're doing from moment to moment is rewarding, deep, and varied enough on its own.