For select query which is better eloquent query or db query? and why?

Actually it doesn't matter in most cases. For example

DB::table('users')->where('name', 'John')->first();

is same as

User::where('name', 'John')->first();

With Query builder you usually use joins and with Eloquent relationships. But with Eloquent you can use joins as well. I prefer to use Eloquent.

There's a bunch of personal preference when it comes to that. I use getPdo (), and of course bind my parameters.

Though I use eloquent for some things.

I stay away from collections, they are so neat when dealing with a few hundred or even two or three thousand records, but you let your database go into the million's, well you don't want millions of Records in an array.