I'm using ConnectX5s for most, and some ConnectX4s in the older servers. Both of those cards have really come down in price in the used/ebay market. I have been playing around with some different optics - I have a bunch of CWDM4s which are very inexpensive and use a single SingleMode pair.... but of course they run hot so if you have them in servers without good air flow you might have problems.
I'm using mostly fiber just because the servers are connected to Cisco 9305 with 72 100g ports.