I have 52 crank and Cassette: 11-28 , i’m using this in Kuwait which is totally flat, this summer i will travel with my bike and climbs in alps! what best crank/cassette to put?
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A general answer would be a lowest gear close to 1:1. that would be a 40 or 42 chainring with a 40 sprocket etc. Alternatively what kind of w/kg do you plan to climb at? That will refine the selection a bit.
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Always have a “get out of jail” gear - I hate going into the easiest gear as I have none left 
around 1.7 w/kg , and i’m around 105 kgs
Get a 50/34 compact chainring set for the front and a 11-34 cassette at the back. That is what I saw the rentals over in Europe do mostly. And practice before hand.
For my taste, a 34 34 combo will spin out too much. Does not suite my style. So something to figure out.
Cheapest way to figure out is to get a cassette, put it onto your trainer, use ROUVY, pick a climb and see what it does. Of course non-ERG and realizm all the way up.
Ah, you need to check which rear derailleur you have. DuraAce only allows 11-30 officially (11-32 if you must). Ultegra has 2 different ones. One called “SS” for a 52/36 11-28. The other one is called “GS”, which will do 11-34.
Most of my heavier friends around here in Colorado do ride a 52/36 11-34. But then again our climbs are usually not steep, just long, and lacking oxygen 
Also get some tools to change the cassette. Easiest way to keep clean anyway, but also it takes not much more than 2 minutes to change a cassette. So if you travel, why not put the 11-28 on for a flat day and a 11-34 for a steep climb.
For reference, around home in Colorado, I mostly use a 52/36 11-28 (spring / autum) or 52/36 11-30 (summer, climbing in Europe).
if you are planning on climbing big European mountains at 1.7w/kg, then you definitely need a 1:1 lowest gear. That will get you up most things as long as you don’t chase stupid climbs with long sections of double digit gradients.
Some awesome advice here guys, thanks.