kind of late to be replying
but my few cents.
-i personally think that the crosscourt against a net player is very risky. it takes a lot of effort to produce a perfect, winning passing shot, especially when on the run. if amount of topspin is insufficient the ball will either land in the net or go long, especially considering the shorter distance on the angled crosscourt shot. if not looking to put the ball away outright and force a weak volley, then maybe, but i still think there are better options.
-a good topspin lob - again, difficult when on the run or without preparation- is the better choice for the crosscourt.
-personally, my favorite is down the line. even when the opponent is covering that side, and the margin for error is low, i still find it so tempting to go for this shot. on the backhand side i prefer a low slice. if you actually have a choice between hitting these two (crosscourt vs down-the-line) shots then most likely you will be in a better position to hit down the line; i just think that it requires less preparation.
-hitting at or very near the opponent can be a very good tactic. but there is a risk; if the opponent net player has very good reflexes and can -like i can
- contort in a split second to get his racquet behind the ball, most likely you're screwed; you'll be stop-volleyed to death!
-one remedy for that last situation is to use as much spin as possible. a hard or medium-strength low slice can do wonders, it has done so for me so many times. it forces your opponent to hit in to the net, because they don't take into account the spin - and even if they did, it would still be a difficult shot when so close to the net (again, it must be hit very low). the downside of the slice is that YOU might hit the ball into the net.
-the alternative shot to the slice when hitting at the net player is the hard-hit topspin. unlike the flat shot, the massive spin will cause the ball to deflect unpredictably if the racquet face of the net player is not ideally aligned. also, by its nature the topspin ball will tend to be at a high point relative to the net player's racquet. both of these facts will work in your favor; you will either get a very weak return volley that you can put away or he will hit the ball out; a stop-volley or anything of that sort is out of the question on such high balls. the downside is that if your shot lacks heavy topspin or maxes out its spin too early and too short, the net player can smash it, angle it, drop-volley it away.
-obviously, as another poster stated, the best possible shot in any situation will of course depend on where you are on the court and where your opponent is.