You can only prevent one from flopping, ensuring all of the planned games release and it's a financial success.

Dreamcast. Not that sega wouldn’t have still dropped out like in the 7th gen.

N64 was a huge success, just not as huge as it could have been because PS1 came out first

64DD didn't really flop per se, it just wasn't supported. They could have decided not to release it but they had a few games ready (the Mario Artist games) so they thought they may as well. There weren't any particularly anticipated games for the 64DD that weren't able to be released on cartridge or reconfigured into GameCube games. So no loss. As for the Dreamcast there were a lot more cancelled games, but, again, anything that was particularly interesting was ported to other systems. The only thing is, the demise of the Dreamcast meant the end of Sega as a console manufacturer... the Dreamcast was also apparently quite good to develop for so it's possible if its life had been extended it might have received some interesting games. But, surely, any good ideas that developers had would have surfaced elsewhere, so it's not a loss. It's like saying, what would happen if the xbox X|S failed today? Well, any games would just come out on PS5 anyway, so it wouldn't matter. In conclusion, I would preserve the 64DD to ensure the end of Sega as a console manufacturer because the market became too crowded for Sega.

N64 because there is honestly almost nothing on Dreamcast for me. Sometimse I download the core on Retroarch just to delete it again 2 days later

I wanna see the N64DD succeed so I can get the f-zero x expansion kit. Most of the dreamcast's biggest hits got ported to other platforms anyway.

dreamcast. i would choose that shit over the proper n64 too.

Dreamcast.
Even though Sega made lots of mistakes in the past, they are missed. Look at what we have these days:
>Microsoft
Has started a lot of abusive and anti-consumer practices, if you're into Japanese games they have pretty much nothing to offer you.
>Sony
They made several great games in the past, but these days they only make games inpired by Hollywood or Netflix and play way too safe. They also closed their Japanese studios even though dedicated Sony fans used to support them precisely because of Japanese games. They also have a serious problem with censorship of Japanese games.
>Nintendo
Endogamic as hell, they make some good games, but they're way too repetitive and safe, always looking for games with a broad appeal and avoiding risks and niches. Countless games starred by Mario and friends instead of creating new characters and new worlds.
Compare all of this with Sega: not only they made several classics, they were constantly creating new series and working with multiple genres and new styles.