Before Sword and Shield I would have said Pokémon. But the franchise went to shit, so today I guess Xenoblade and Metroid take this.

2D Mario. It's what started it all and I still enjoy the heck out of it, even moreso now that it's up to 4 players.

Mario
Super Smash Bros.
Animal Crossing
The Legend of Zelda
Fire Emblem
Mother (EarthBound)

The Legend of Zelda always has, and always will be my favorite Nintendo IP.

That said, Xenoblade Chronicles has certainly pushed its way to number 2 in my book.

Mario and Super Smash Bros.

Tough choice

5. Mario Kart
4. The Legend of Zelda (2D/isometric titles only)
3. Tokio Mirage Sessions
2. Fire Emblem
1. Earthbound

Mario for sure. Every outing is fantastic and almost always different from the last one (only Galaxy and Galaxy 2 were the same kind of experience - but both were incredible with Galaxy 2 maybe being the best Mario ever).

Mario Kart (not listed for some reason - unless OP just meant to include it with "Mario universe") and Smash are always amazing but the experience is always extremely similar of course.

Metroid is great but there have been two 8 year and currently a 13 year break between installments (on consoles) in the series so it's a much less prolific series.

The only main contender to Mario for me is Zelda. Though I realize now that most Zelda games I haven't even finished (though hopefully that'll change with a good Zelda collection next year), while I've finished every single Mario game except SMB 2, and the two I never owned: NSMBWii & 3D World (as far as console Mario's go). And BotW is the first time I've been really excited about Zelda since Wind Waker. I'd say Zelda is the more exciting series right now because BotW really blew things up and we're all waiting on BotW2 to see how Nintendo ups the ante again, but overall Mario easily wins this for me.

1. pokemon 2. Mario kart 3. Smash 4. Animal Crossing 5. Chibi Robo

Based on their consistency, it would be these.

1. Super Mario

2. The Legend of Zelda

3. Kirby

4. Metroid

5. Style Savvy

6. Donkey Kong

7. Pikmin

8. Wario Land/Ware

Only a handful of exceptions to these (namely just Lost Levels and Other M) that I don't enjoy. The rest range from really good to excellent.

Pokemon is up there, is a game series that I'll play most games if I can, even if I don't care for gen 2, 4 and 6 much.

Fire Emblem would have taken number 5, but since Awakening, my interest and care for the series have gone downhill significantly.

Yoshi needs more great games, besides Island, Woolly World and Touch & Go.

Zelda, easy. I just never get tired of experiencing what they do with this franchise.