Depends on how cost sensitive you are. Currently the LimeSDR[1] is the best value (even at $300 and no case) because it is not only full duplex, it also supports MIMO operation. While the latter is intended to support things like 4G LTE it also gives one the ability to do direction finding as multiple receivers clocked by a phase locked LO allows you to do phase analysis of incoming signals.

The Ettus USRP B-series[2] is very well supported for about $1000.

The ADALM-PLUTO[3] at $150 is a bargain for a full duplex SDR but a range of 325 MHz to 3.8GHz leaves a lot of interesting spectrum out. That is less of a problem if you get a mixer and RF signal generator because you can up/down convert the HF/VHF/UHF bands up to 1 GHz as an IF and play with them there. There are some off the shelf solutions for this like the 'Ham it Up'[4] but you are at the mercy of their TXCO.

I have all of these solutions except for the USRP and find that when I reach for a radio to look at a signal I usually grab the LimeSDR.

[1] https://www.crowdsupply.com/lime-micro/limesdr

[2] https://www.ettus.com/product-categories/usrp-bus-series/

[3] https://www.digikey.com/products/en?keywords=PLUTO

[4] https://www.nooelec.com/store/sdr/sdr-addons/ham-it-up-304/h...