what race and why?

I'll give a very abbreviated breakdown. Each race has a depth of nuances that could be argued one way or the other as to their viability.

Human - Well rounded and versatile race. Most the necessary heroes are free to obtain, i.e. Virion and Avalon. They can siege well, clear timed events, and are decent at PvP.

Lich - Very PvP oriented race, but a bit of a one trick pony when it comes to playstyle. They struggle with sieging compared to Dwarf and Human, but with adequate research in skeletons they are viable at end game. They do great in challenges that require no deaths or minimal heroes, but struggle with speed clears.

Dwarf - Fun offense oriented race with one of the best frontline units in the game and great burst damage. They siege well, PvP well, and can clear timed events. Their biggest downside is the significant RSS loss when something doesn't go your way in a siege or PvP. This is also true if someone manages to clear your garrison, you can easily lose tens of thousands of rare RSS worth of troops. High risk, high reward.

Rakan - I don't play Rakan much. They are phenomenal at PvE speed clears, and I've seen them be viable in PvP when they can activate their abilities to clear backlines. They are very elixir dependent and can use tens of thousands of it per battle. Their troops also loan well for void.

It depends on your style of play and the heros you have. Dwarf and human are the most fun and the easiest to play. They steal many resources but their defenses are very vulnerable. Lich and Rankan are good at protecting cities, but a Rankan is vulnerable if not protected by another Rankan. They steal less resources and it is very difficult to grow.

I think it all depends on buying heroes... Playstyle don't make difference . I have seen rakan do just as well in bronzing up to lvl 11 troops or even pure lich taking out 60 golds garrison...

As someone who has played Rakan since it was released and is now playing at max level with it do not choose Rakan as your main race. You will be able to do hardly anything by yourself, you will have a resource burden no one else has inside and outside of void mirror (I've probably spent 9-10 million at least in void mirror over time) you will constantly get loan requests for crossbows that will become a headache, you have to trade defense with another rakan to have any decent defense, your samurai will lose to human swords due to virion Rez, your monks will lose to human swords due to running away, everything you can do will lose to human swords because all anyone borrows is human swords. Your rakan specific heroes will have no +1 ability items, samurai have no buffs from any hero at all. In short it's frustrating garbage to try to do anything on your own.
Pick human and focus swords, pick dwarf and focus blasters, or pick undead and focus skeletons/spiders. Then ignore anything else and borrow swords as dwarf, blasters as human, or just level rose to 50 and annihilate armies with silver spiders from a distance

Rankan needs turtles in the wall if he is not protected by another Rankan. The maximum number of turtles that can appear are 3 in 1 attack. 3 turtle bonus a lot.