The lowest I'd go would be Tiagra. In fact I have one bike with 9-speed Tiagra that I use on a rails-to-trails and it works fine. Very few shifts on a flat trail though.
My road bike has 105 10-speed and it works very well indeed. Back in the day I've had Ultegra and Dura Ace groupos. Today's 105 is as good as they were.
As a recreational/century rider now, I really can't see going much higher than 105 and the idea of electric shifting won't happen unless the price comes way down or I win the lottery.