.ssh/config to start remote session in zsh

I don't think that is possible with ~/.ssh/config. The -t can be covered by adding a RequestTTY yes, but it doesn't seem you can specify the remote command in ~/.ssh/config.

However, with zsh, you could add a:

alias -g 'serveralias=serveralias -t zsh'

to your ~/.zshrc.

Or make a function like:

zssh() ssh "$@" -t zsh

For anyone arriving here years later, since 2017 it's possible to put everything in the config file:

Host host_1
    HostName 1.2.3.4
    User root
    Port 22
    RequestTTY yes
    RemoteCommand zsh

I concur with @Stephane that there isn't a way to do this using the ~/.ssh/config file. Another approach would be to use the ~/.ssh/authorized_keys file on the remote server. If you add a line like this:

command="exec zsh" ssh-dss ..... rest of key ....

Then you can just ssh as normal and you'll get a zsh on the remote server.

Example

On server, ssh to remote.

$ ssh saml@greeneggs

On remote server, confirming we're in a zsh.

[saml@greeneggs]~% ps -eaf|grep $$
saml      1974  1973  1 10:34 pts/3    00:00:00 zsh
saml      2023  1974  0 10:34 pts/3    00:00:00 ps -eaf
saml      2024  1974  0 10:34 pts/3    00:00:00 grep --color=auto 1974

You can do more elaborate things using this file, see this Q&A, titled: ssh, start a specific shell, and run a command on the remote machine?.