Firstly, I hate the idea of crossovers with my beloved Trek.
However, if you forced me to pick something, the only one that comes to mind as something barely watchable is:
On TNG, to have the wonderful Jeremy Brett as Sherlock Holmes. I'd happily have some kind of holodeck story play out, so long as I get some scenes where the two greats Brett and Stewart share screen time together. Seeing Spiner play off of Brett and of course with David Burke/Edward Hardwicke helping out just sounds like a lot of fun.
I think this is the best style of crossover as it wouldn't be too much of a stretch and wouldn't be too hard to make it gel properly.