Which flasks are worth to make into Blue Magical?

I got a +35% chaos resist flask. worth making it into a blue magical?
if not, then what are?

and what modifiers do i want on these flasks?

Flask you will be using are worth modding and quality-ing

All of them you'll be using... if you can afford the transmutes/alterations.

When you first start out it's probably best to leave the lower level flasks however you found them and save your transmutes for weapons and rings. You'll be changing flasks pretty quick early on. Good flasks help tremendously though, so don't be afraid to use currency on them if you can spare it. Keep your eye out for superior flasks with quality, but I wouldn't use baubles on anything but the highest level flasks (level 53).

Utility flasks like Amethyst are a bit different as there are not higher level versions of them. Any flasks you find in act 2 onwards can get all the flask mods in the game. Feel free to invest more currency into these to get what you're looking for as you can use them for the whole game. In the case of a quicksilver (movement speed) or granite flask (armour) it's best to use baubles on them as a white to bring them to 20% quality before making them magic.

Useful mods

Life (prefixes)
* Instant healing. Always have at least 1 of these ("Seething", "Bubbling")
* More healing ("Saturated")
* Faster flask refilling ("Perpetual" or "Surgeon's" for crit builds)
* "of Heat" removes freeze/chill
* Suffixes mods can be anything useful ("of Ironskin", "of adrenaline", "of resistances", etc)

Mana
* Same as life except I don't find any of the instant flasks useful
* Movement speed ("Adrenaline") is very useful same with "of resistances"

Quicksilver (20% quality)
* Perpetual/Surgeon's" of Adrenaline
* "Ample" may also be good for extra charges

Granite (20% quality)
* Perpetual/Sergeon's of Ironskin (extra armour)
* "of Heat" can also be good for freeze removal
* Curse removal ("of warding")

Resistance (Amethyst/Ruby/Sapphire/Topaz) 20% quality
* Perpetual/Sergeon's
* Curse removal ("of warding")
* Or any useful suffix

These have specialized uses, mostly for boss fights or against elemental reflect. The chaos flasks are good for zones with snakes, poison arrow archers, or poison trap throwers and are a good option instead of (or in addition to) taking chaos resist passives. Keep them in your stash and swap them in as needed.

EDIT: curse removal ("of warding") can be very useful as well, especially for resist flasks (read elemental weakness from necro's in chamber of sins)