Slice in rec doubles is the best return, I'm not joking, unless your topspin is 90% accurate and reliable. Those with good slices like me, I can return any serve low and crosscourt at 4.5. 5.0 I have no idea.
I think one of the best drills for doubles is this...Points are only rewarded if your return and subsequent taking of the net result in a winner OR if your serve and volley result in a winner, no points are given for winner returns or winner serves. Yeah, I know, some outliers that have bomb serves should use that weapon. Anyway, it's a good way to teach you to use the serve and use the return as a tool to create a situation where your team has the opportunity to hit a shot at an advantage. A serve or return winner is probably a 20-30% chance of happening. But, a good setup serve or setup return can happen 80% of the time and then the next time your team hits the ball you are in a position to win 70% or so of the points. So, do that over and over, and you win your matches.
In singles, I warmup with some deep slices to the opponent, if they seem surprised or uncomfortable with my lefty slice spin crowding their open stance forehand by driving into the throat of the racket, I'll use it a ton in singles and it works. Especially against the teenagers these days, I'm not sure they've ever seen a slice, literally.