If you had to flip One Senate Seat within 10% to the other party in every cycle

[Schiff for Senate]

2020: Ronchetti (NM) I guess? I don't know a whole lot about him, but all other Democrats said him, so presumably he's a saner or more moderate Democrat? That and the fact that Lujan had a stroke and it'd perhaps be better if he lost and didn't have the stress of being a senator
2018: Definitely Morrisey (WV). Manchin breaks with the party on enough stuff - understandably so - and it's a Trump+40 state anyway, so in 2024 it'll go red regardless. Makes sense for a Trump+40 state to have a GOP senator, and anyway Manchin is a wild card on enough things. I might've considered flipping NJ given that Menendez is totally corrupt, but it wasn't within 10 points anyway.
2016: Ayotte (NH), probably. Only 3 options here anyway.
2014: It's either Gillespie (VA) or Brown (NH). I'll go with Brown. On both the Democratic and Republican sides, it seems NH would have the better candidates though.
2012: Berg (ND), for the same reason as Morrisey in 2018.
2010: Rossi (WA). Punish Murray for her insider trading.
2008: Coleman (MN), for obvious reasons.
2006: Kean (NJ) for sure. Corrupt Bob doesn't belong in the Senate, and Kean is clearly one of the better and more moderate/pragmatic Republicans.
2004: Only one choice here...Coors (CO).

[Schiff for Senate]

2020: MJ Hegar defeats Cornyn
2018: Beto O'Rourke defeats Cruz
2016: Feingold defeats Johnson
2014: Kay Hagan defeats Tillis
2012: Rich Carmona defeats Jeff Flake
2010: Joe Sestak defeats Pat Toomey

TO THE OTHER PARTY. So, if you’re a Democrat, which presumably you are, you have to flip one seat Republican in each cycle. For instance, in 2018, you would have to flip one seat Republican, so you could choose to flip WV and have Morrisey win rather than Manchin.

There is a thread for the converse (flipping 1 senate seat to your party in each cycle), which is what you did in your post, though: https://talkelections.org/FORUM/index.php?topic=518575.0

[Schiff for Senate]

Keeping tally for what Democrats said:

2020: 80% NM, 20% CO
2018: 80% WV, 20% AZ
2016: 100% NH
2014: 80% NH, 20% VA
2012: 60% ND, 20% IN, 20% MA
2010: 60% WA, 20% CA, 20% DE
2008: 60% MN, 40% LA
2006: 80% NJ, 20% RI

[Schiff for Senate]

2020: Ronchetti
2018: Hugin
2016: Ayotte
2014: Brown
2012: Brown
2010: Rossi
2008: Coleman
2006: Kean
2004: Coors
2002: Thune

Menendez won by double digits, so this answer isn't valid.