Won Kennedy the election, but Johnson would probably have ended up in jail if Kennedy had not been shot.  As president, he skuttled all the investigations into his crooked dealings and blundered into Vietnam.  At least part of the Vietnam stink clings to Kennedy.

LBJ was a great choice for a running mate in the sense that he had the machine to deliver Texas's electoral votes. Since it came down to the smallest of margins, this was important. If he had anything to do with JFK's murder (one of the many theories), then he was a really bad choice.

Good strategic choice, providing a southern voice on the ticket and a healing of the primary campaign battle. Kennedy and Johnson didn't like each other, but managed to put gloss on that during the campaign to help squeak out a narrow victory.

Johnson hated Kennedy more than did Richard Nixon but politics makes strange bedfellows.  Hungry for power, after waging a nasty primary campaign, Johnson accepted the VP under Kennedy.  If there was one person Johnson hated more than Jack, it was little brother Bobby, the Attorney General.  It was a combination that worked, if just barely, winning by the absolute narrowest of margins, as Johnson placated the conservative democrats of the South at that time and Kennedy appealing to the big city/industrial states' union workers.