Best strictly side-scroller: I really
did like Bad Dudes vs Dragon-Ninja, although RoboCop and Rolling Thunder get honourable mentions.
Best more-or-less side-scroller: Final Fight, hands down. One of the best
games ever,
IMO. I think it was one of the first of this type (obv Double Dragon was older but that game blew huge chunks), and inspired the side-scrolling-plus-up-and-down-the-screen genre which included some real gems like X-Men (the huge double-wide six player version was my favourite

)
Viperion