I have absolutely no doubt that Jeanne Shaheen would have been the best pick. She would have sewn up New Hampshire, provided enough distance from Clinton to probably win Tennessee, reassured enough women and excited enough members of the base to raise Gore's margin and prevent Nader defections in who knows how many states, and done no harm in Iowa or Wisconsin.
Gephardt would've been a good pick for him. I don't know that much more could've been done for Gore. For as much as he trailed Bush in 2000, it's actually pretty impressive he did so well. On the other hand, it could be argued that if anything had gone right for him, he would've won. He managed to cling onto every single one of Clinton's negatives and suffer from association, but still couldn't be credited with any one of his positives. It's even simpler when looking at the gun rhetoric which could be argued to have cost him his home state of Tennessee, Clinton's home state of Arkansas, long time Democrat state of West Virginia, the swing state of Ohio, and even the long time swing state of Missouri. Florida wouldn't even have mattered if Gore would've won his home state. Gephardt should've won him Missouri. I can't see Daschle on a national ticket though. Grey Davis wasn't as unpopular at the time.