Assuming we get the best versions of each player, and that we’re playing a team tournament against teams made of the guys we don’t choose, across surfaces:
Big 3 - Nadal. Instant win on clay against the rest of the competition, so you’re guaranteed 1 win. Hard to make the Borg comparison, but that might be the only wildcard depending how much era adjustment you want to allow for.
Now, assuming the other teams are smart enough not to waste their best players against Nadal on clay, you need some other options to counter the other greats on their best surfaces.
Cat 3 - I pick Sampras, to go against Roger at Wimbledon, or against the other legends on fast to medium fast surfaces.
Cat 4 - these guys are all mental midgets and unreliable, so I’ll go with Nick who gets a motivation boost from playing in a team setting. He also gets fired up for big matches against the big 3 or other next gen.
Cat 5 - I would pick Stanimal to put him up against Djokovic on plexicushion. At least that way you have a chance to win that matchup. Hate leaving peak Delpo on the table, and probably end up giving up a loss to the other team playing against him with one of your weaker guys.
Cat 2 - hardest choice for me is which Nextgen to pick. Since I’ve already picked Nadal I’ll pass on Thiem, who is improving elsewhere but seems to have a hard time closing the deal in big matches. Medvedev fall 2019 was probably the best level produced here but I have Sampras and Stan that can play big matches on HC, and I already picked Stan to counter Djokovic which would be Med’s other argument. I would probably pick Stef. He’s not a guarantee in any matchup, but he’s pretty good on both HC and Clay and I think has the game to get better on grass. He’s been able to beaten admittedly declined versions of each of the big 3, making him a decent B team choice. He likely takes a loss if he has to play Medvedev though so this is the toughest call.
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If I started with Djokovic, I take Stanimal off the board, Pete to take on Roger on grass, give up a win to Nadal on clay, pass on Nick for Monfils since they don’t get along, and probably take thiem amongst NextGen to take a swing at Rafa.
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If I started with Federer, take Nick who will be fired up to be on my team, take peak Delpo off the board who usually plays me hard. Tough call is then whether to take Pete off the board to give me a near guaranteed win on Grass, or trust in my ability to beat him and take Borg to take a swing at Nadal. If I take Pete, then I take Thiem from next gen to play on clay. If Pete, you have a pretty good chance on both grass and fast HC. If I take Borg, I take Medvedev to give me an option against Djokovic since I left Stan on the table.