Depends on the campaign, players, and circumstances.
I've done the 'meet in the tavern' thing.
I've started a campaign with the characters serving as caravan guards, and/or travelling with the caravan. It gets ambushed, and a fight ensues. That first fight is the 'meetup' for those characters whose backstories don't already dictate that they knew each other.
When I started a group of mostly brand new players with the Sunless Citadel, their characters were diverse enough that I had them hauled before the local lord, having just been released from jail, but still owing fines due to various mishaps around town they'd been involved in, accidental or not. Taking on the quest was their alternative to being shipped off from Five Oaks in the Gnarley Forest to Greyhawk in chains.
In Saltmarsh, I had the starting players staying in various places around town- the mage character was billeted with Keledek, doing research and scribe work in return for a place to stay. It didn't hurt that she was a half-elf with high charisma, and Keledek was attracted to her- she didn't know about his evil alignment. Her brother (another player) was an aspiring knight (Fighter, planned cavalier-type) who was billeted with the guard in exchange for work as a town militiaman. A dwarf fighter with the masonry background was working for the mines, the party rogue was a privateer, first mate on a ship recently arrived in Saltmarsh- sent by the Keoland navy, they were hunting smugglers, but nobody in town knows this. An elven ranger and elven druid from Silglen in the Dreadwood were the two 'out of towners', with vague backstories to start but they somehow ended up in Saltmarsh. The party all basically met each other role-playing about town, and then banded together to go hit the haunted house. This was a bit convoluted of a start- had any one of those characters not survived the first session or two all that groundwork would have been something of a waste. Don't know if I'll do this sort of thing again- it seemed like a good idea, but the whole first session was a big muddle-around with a split party, and not very engaging in hindsight.