2014 Giant Defy Advanced had 10 Speed 105 with a 50/34 chainset and 11-28 cassette (11,12,13,14,15,17,19,21,24,28)

The 50x15 gives a ratio of 3.33 and the 50x17 gives 2.94.

You can get a Tiagra 10 speed 12-28 cassette from here:
Shimano Tiagra HG500 10 Speed Road Cassette | Chain Reaction Cycles

which will give you that 50x16 with a 3.13 ratio.

This website gives you the full list of ratios for the 50x34 and 12-28:
HTML5 Gear Calculator

According the Giant website the Defy Advanced 2 for 2015 has 11 speed and disk brakes while the 2014 has 10 speed and rim brakes.

Now, if you have the 2015 and 11 speed 105 by some chance then things become trickier as there is no real easy way to get a 12-28 11 speed cassette. However, it does have the Shimano RS500, 34/50 crankset which will be easier to fit on a smaller big chainring as they are a standard 5 bolt design rather than the specific Shimano 4 bolt design.

A 48T big ring by 15 at the back will give a ratio of 3.20. Thing is, 48T 110BCD chainrings are not common. In fact they are bloody rare. I don't think Shimano actually makes one but I could be mistaken on that.

A 46T big ring would be no help to you as the 46x15 is 3.07 and then 46x14 is 3.29 which are both close enough to the 50x17 (2.94) and 50x15 (3.33) as to make no difference.

If you do have an 11 speed bike I'd be tempted to get a 12-25 cassette and take the bottom half and stick it onto your 11-28 cassette. That would give you the golden 50x16 but it does mean needing to buy two cassettes.

I hope for your sake you have 10 speed as the Tiagra 12-28 10 speed would be the cheapest option.

EDIT: It appears the Tiagra 10 speed does not have a 16T. So you would probably be better off with the 105 12-27 or taking the 105 12-27 and mixing it with your existing 11-28 if you need the 28T.