Best places to leave an alt that has a bank/mailbox? Hands down - Thunder Bluff (easy access to mailbox/bank/AH/Forge & Anvil). I really, really, really don’t like trekking halfway across the city to the blacksmithing/engineering trainers and the forge (like Orgrimmar and Stormwind), and realize I forgot something in my bank, or need to buy something from the AH. Thunder Bluff is a great place for characters to hang out while earning well rested bonus, since it’s in the middle of the map. It’s also nice to leave a character who’s a miner, so they can smelt ore for your engineers/blacksmiths/Jewelcrafters if they happen to not be miners.
On the Ally side, Exodar (Draenai capitol city) is probably the best. It’s way out of the way, but people who want to log on just to check prices or get their mail, the mailbox is right by the ramp from the AH, making it easy to get your unsold items, and repost them quickly, without having to swim/run to the Auction house from the mailbox, like Darnassus. Darnassus would be the best place to leave a tailoring alt, so you can fly to Auberdine every 4 days to do a Mooncloth transmute. (Check in the Park in Stormwind, it looks like a Moonwell is there by the druid trainers). For Horde, Orgrimmar is the best place for a tailoring alt, since you can quickly fly to Ashenvale to do your Mooncloth transmute.