Both iSSH and TouchTerm are good choices - they have pretty similar functionality but a few slight differences, mostly comes down to personal preference as either will get the job done.
One recommendation, it pays off to plan ahead before you're confronted with an emergency and setup some scripts or aliases for common commands, especially if you need to login to multiple machines to reach backend servers, etc. While the onscreen keyboard is workable, it's still much slower than typing on a real keyboard. Also, make sure to spend some time experimenting with the app's functionality before having to use it under fire, so you aren't trying to figure out how to make it work while your servers are down.