Is it better to level up higher rank servants first?

In my own opinion:

My quick answer: Focus 5 stars.

My long answer: Pick a favorite servant from each class you like and focus on them regardless of stars. At the end of the day, you can clear story content just with the 3 star servants.
If you're really unsure, I'd cross check. This GP tier list is not the definitive list. There are servants on here that are rated tier 3-4, but on other sites or YouTube channels the servants are rated a lot higher and vise-versa.

If you're a new player max ascend them; otherwise pick up 1 ST & AOE servant for each class (ignoring stars) and focus on them.

Usually rise one all target and one single target np from regular class.
I also recommend raising the following servants. (and also mash)
Saber: Caeser if u don't have a single target saber from sr/ssr,
Bedivere if u plan to np5 him. (even at np2 is a solid choice) on NA he is missing his interlude.
Archer: Euryle, Robin and David are Frequently used archers even when one has high tier Sr or ssr from this class.
Arash is basic of most 3turn strategies.
Lancer: normal Cu for survival,
proto for crit.
Jaguar Man if she is np2 or higher
Rider: Ushiwakamaru (has quite a few events where she is a bonus servant, has a great np damage so she is a great budget option too, her kit is also supportive. Mine is almost bond10) and also the only single target damager in the low star category,
Medusa is great for daily farmings (20% charge without skill leveling),
Georgious as a tank,
Alexander can be a (makeshift) quicksupport.
Caster: Medea her debuff clear from single ally and her buff clearer np that can be looped makes her a great candicate for challenge quests helper (also the only low star single target caster),
babbage is great for all target np caster, also durable.
Hans(general/crit damage/star gen/hp regen), Mozart(arts/50 crit star maker skill) and Sheakespear (buster/20% np charge/all enemy targeting np with a chance to stun them)
Assasin: Any of the hassans are usefull for damage Hundred Face is the better option in the long run. Relying on Poison damage will be the best with Serenity. Long arm is the best at Star gen (in the lower rarity) also has similar evade as Cu lancer
Jing Ke also has a nice damage and crit damage.
Fuuma has Targetable evade granting skill and it's good at star generating by skill.
Berserker: lubu the single target monster has (almost) the highest np damage (sr/ssr are at np1-3 at this comparing)
Koyohime is capable of ereasing her targets with her and also has good np gain.
Spartacus: self np charge+guts, good np gain and relativly sturdy with many healings (has a regen skill, a simple healing+buster up and a np that heals him) used mainly for farming

Short answer is work on the 5*s, they have the best stats.

Long answer is that 5* servants are too expensive to field en mass, so it is a good idea to have a decent 3 and 4* servant to fill the party more cheaply, especially for events when you need to field as many 5* CE's as possible to farm effectively.

Short: whatever you want.
You can get very far in the story simply by having a strong support servant from your friend list.
A bit longer:
I would suggest leveling up Mash first. She is pretty useful in protecting other servants, and it's a very big bonus that her cost is 0.
Beside her, I would suggest leveling at least one servant of every class depending on who you have, and who you like.
In general, the more stars, the higher the potential of the servant, and you really don't need to worry about which ones are good early on. But there are outliers in both directions. If you want to go with usefulness, a good 3 star can be a lot better than a bad 4 star (or even 5 star). If you like them, don't care about it, and go for them. You can start worrying about a strong team comp later.

For me, it's whoever I want to use the most. I think I got Iskandar shortly after I started about 11 months ago but I found him a pain to use so he's still just L30 or so . Plus, I don't usually use class-specific EXP on non-matching servants, so I don't really target one servant anyways, except for the wildcard EXP. Each class more or less levels up in parallel, but sometimes ascensions have to wait their turn for mats.

no, better to focus on the most usefull one.
and usually is the 5 star.
but not alweys. For exemple, if you got a medb, there is no real reason to invest on her right of the bat instead of ushiwakamaru.
Simply invest on what you need the most at the moment. and meaby take a look on what you may need in future event .

i say this but i am leveling edmond dantes instead of helena despite i need her more then him

Depends on you really. In JP server, Salter is my main saber , but then I later got Mordred. But Mordred stayed at lvl 1 for several months and I grailed Salter to 90. She's almost bond 11 now.

But then again, Salter is high tier damager, so you might want to look at their usefulness as well. For example, prioritizing Ushiwakamaru before Astolfo or Marie Antoinette.

Focus on the servants you like first. Then focus on utility each servant brings.

Ushiwakamaru has the highest ST damage of all Riders on her NP outdamaging even 5 stars. Andersen is the best free support in the game and his only real competition are 5 stars. Billy the Kid has also ridiculously high NP damage and Crit damage.

Remember that teams have a max cost so having lower rank servants is a very good thing.