There are so many choices for transponders right now. You need to be thinking about having ADSB out too. I have a remote Garmin GTX-23ES transponder and I added the Garmin GPS-20A WAAS GPS to make it ADSB compliant.

If was doing a new panel I would be thinking about the Garmin GTX-335 (all in one) or a used Garmin 327 or 330 with a sniffer type device for adding the ADSB out; Garmin and Uavionix have reasonably priced units for doing that.

The best advice I got on avionics is to not buy anything till you are ready to install them. Things are changing so fast, every few months new stuff comes on the market.