The Supercard line is the worst choice for GBA games. Many games have excessive slowdown or glitches, and some games just don't work at all.
The Elink is an old-style flashcart. It requires you to use a Windows program flash the games to the cart, it doesn't have a lot of space for games, writing games is slow, and managing multiple games on the cart or dealing with save files is a pain. Aside from those issues it's not a bad cart. It is cheap, and it has real time saving.
The 3-in-1 and the EZ-IV are very similar. The main difference is that the 3-in-1 is an expansion pack (meaning that all files are stored on a slot 1 flashcart and only copied to the 3-in-1 when they are needed) whereas the EZ-IV is a standalone flashcart with its own loader and memory card port. They both have excellent compatibility (aside from a problem with sleep mode), but not a lot of extra features (no real time save).
Out of those carts, I'd personally go with the 3-in-1 if it was going to be used in a DS (this is what I use), or the EZ-IV for a GBA.