without hesitation, I answer that Kirby question with Kirby Superstar (Ultra) and Kirby Air Ride. With Return to Dreamland at a close Second for Main line, and... I sadly haven't played Planet Robobot because I'd probably crown that third from what gameplay of that game I've seen.
Canvas Curse and Mass Attack are spin-off titles.
I would consider Mainline kirby games to be games where Kirby possesses a variation of the control scheme he does in the original Kirby's dreamland, and that kirby has top billing in (to avoid any NES Remix product, Let's Tap! Amiibo, or Super Smash Bros. games from qualifying in this aspect)
The games that fit such a formula are as follows.
Kirby's Dreamland
Kirby's Adventure
Kirby's Dreamland 2
Kirby Superstar
Kirby's Dreamland 3
Kirby 64: The Crystal Shards
Kirby Nightmare in Dreamland
Kirby and the Amazing Mirror
Kirby Squeak Squad
Kirby Superstar Ultra
Kirby's Return to Dreamland
3D Classics: Kirby's Adventure
Kirby's Dream Collection (Mainly included for the Challenge mode)
Kirby Triple Deluxe
Kirby Planet Robobot
Note that Team Kirby Clash Deluxe and Kirby Fighters Deluxe are disqualified because they do not feature Kirby's ability to inhale enemies, which is a control mechanism from the original Kirby's Dreamland.
Although the likes of Epic Yarn, Mass Attack, and the Curse games have a similar level structure and have varying levels place in the Kirby canon, I'd argue that the inclusion of these games into the core lexicon opens you up to having to include Kirby Tilt n' Tumble, Kirby Block Ball, and all the Aforementioned games that kirby does not have top billing in as "Mainline Kirby Games"
God knows that Subspace Emissary is a heck of a lot closer to a Kirby game game flow wise than any other game represented in Super Smash Bros... and really, Kirby ends up being the one who saves everyone after Tabuu does his impression of Gill from Street Fighter 3.