To understand the film, none.

To understand the jokes, you need to watch or know [X-Men Origins: Wolverine][1] and [X-Men: Days of Future Past][2].


Those jokes and all the other references (almost) are listed in [this Movies & TV answer][3]. (It’s full of **spoilers**.)

Colossus's character did appear in X2, X-Men: The Last Stand and X-Men: Days of Future Past, but none of his past roles have any importance for this film and he is even played by different <strike>actor</strike> actors. 

<sub>Note: There are many non X-men film jokes/references too like Green Lantern, Taken (film series) etc etc which you can check from above Movies and TV answer.</sub>

  [1]: https://en.wikipedia.org/wiki/X-Men_Origins:_Wolverine
  [2]: https://en.wikipedia.org/wiki/X-Men:_Days_of_Future_Past
  [3]: http://movies.stackexchange.com/questions/48786/what-were-all-the-references-in-deadpool