There are generally only 2 options to counter an opponents topspin shot.
- Hit topspin back (the aggressive tactic)
- Put underspin on your shot and "slice" it back (slow it down by putting lots of opposite-direction spin on your shot). This would be the defensive, controlled tactic.
A heavy chop serve is entirely different. You have to get under the ball enough so that you don't drive it into the table (or net), but not so much that you pop it up (for an easy smash by your opponent) as well as not putting too much forward motion into your shot that you hit it long/deep off the end of the table. Depending on how hard of a chop your opponent hits on the serve (and how much back or side spin the server imparts on the ball) this can be pretty tricky and it will take a fair bit of practice before you can consistently hit a good neutral return - especially if your opponent is good enough to alter the direction of the spin each time.
Edit: If you watch some video clips of professional table tennis - you'll notice that they generally hit a "chop" style serve, trying to put their serve as short and low over the net as possible - this gives them the biggest advantage because:
- Their opponent has to move up quickly and become out of position potentially for the next shot.
- Their opponent has to hit up on the ball, creating enough margin between the height of the ball and the top of the net to enable them to hit a fairly aggressive shot on their next shot, allowing them to become the aggressive player that dictates the outcome of the point.