The one that I use for now, always in constant change... (about to add PostCSS)

gulpfile.js

var autoprefixer = require('gulp-autoprefixer');
var browserSync  = require('browser-sync');
var concat       = require('gulp-concat');
var del          = require('del');
var gulp         = require('gulp');
var htmlmin      = require('gulp-htmlmin');
var kit          = require('gulp-kit');
var merge        = require('merge-stream');
var minifycss    = require('gulp-minify-css');
var nano         = require('gulp-cssnano');
var newer        = require('gulp-newer');
var plumber      = require('gulp-plumber');
var pngquant     = require('imagemin-pngquant');
var postcss      = require('gulp-postcss');
var rename       = require('gulp-rename');
var runSequence  = require('run-sequence');
var sass         = require('gulp-sass');
var sourcemaps   = require('gulp-sourcemaps');
var uglify       = require('gulp-uglify');
var uncss        = require('gulp-uncss');

var SRC          = 'src/';
var DST          = 'dst/';


/****************************************************/
// Ejecutar "npm install" para instalar dependencias
// y luego ejecutar "gulp build" para ejecutar todas
// las tasks que volcan en la carpeta DST
/****************************************************/
gulp.task('build', function(callback) {
	runSequence('clean', ['images', 'scripts', 'fonts', 'favicon', 'robots', 'htaccess', 'sitemap', 'htmlkit'],
		'htmlmin', 'sass', callback);
});


/****************************************************/

/****************************************************/
//
// 					DEV MODE
//
// Watch para desarrollar con actualización de cambios en sass y html
//  28 Oct: no se si els js s'estan copian bé. He de cridar a totes les funcions, com faig a 'build'?
/****************************************************/
gulp.task('watch', ['watch-sass'], function() {
	browserSync.init({
		server: DST
	});
	gulp.watch(SRC + '**/*.kit', ['watch-kit']);
	gulp.watch(SRC + 'css/**/*.scss', ['watch-sass']);
	gulp.watch(SRC + 'js/**/*.js', ['watch-scripts']);
	gulp.watch(SRC + 'img/*', ['watch-img']);
	gulp.watch(DST + "*.html").on('change', browserSync.reload);
});


gulp.task('watch-sass', function() {
	return merge(
			gulp.src(SRC + 'css/estilos.scss')
			.pipe(sass())
			.pipe(uncss({
				html: [DST + '*.html']
			})),

			gulp.src(SRC + 'css/looper.scss')
			.pipe(sass()),

			gulp.src(SRC + 'css/tooltip.scss')
			.pipe(sass()),

			gulp.src(SRC + 'css/acordeon.scss')
			.pipe(sass())

		)
		.pipe(autoprefixer())
		.pipe(concat('estilos.css'))
		.pipe(nano())
		.pipe(gulp.dest(DST + 'css/'))
		.pipe(gulp.dest(DST + 'css/'))
		.pipe(browserSync.stream());
});

gulp.task('watch-kit', function() {
	return gulp.src(SRC + '**/*.kit')
		.pipe(kit({
			compilePartials: true
		}))
		.pipe(gulp.dest(DST + ''));
});

gulp.task('watch-scripts', function() {
	return gulp.src(SRC + 'js/**/*.js')
		.pipe(plumber({
			errorHandler: function(error) {
				console.log(error.message);
				this.emit('end');
			}
		}))
		.pipe(concat('main.js'))
		.pipe(uglify())
		.pipe(gulp.dest(DST + 'js/'))
		.pipe(browserSync.reload({
			stream: true
		}));
});

gulp.task('watch-img', function() {
	return gulp.src(SRC + 'img/**/*')
		.pipe(newer(DST + 'img/'))
		.pipe(pngquant({
			quality: '60',
		})())
		.pipe(gulp.dest(DST + 'img/'))
		.pipe(browserSync.stream());
});

/****************************************************/
/****************************************************/



gulp.task('clean', function() {
	return del([DST]);
});

gulp.task('images', function() {
	return gulp.src(SRC + 'img/**/*')
		.pipe(pngquant({
			quality: '30',
		})())
		.pipe(gulp.dest(DST + 'img/'));
});


gulp.task('scripts', function() {
	return gulp.src(SRC + 'js/**/*.js')
		.pipe(plumber({
			errorHandler: function(error) {
				console.log(error.message);
				this.emit('end');
			}
		}))
		.pipe(concat('main.js'))
		.pipe(uglify())
		.pipe(gulp.dest(DST + 'js/'));
});

gulp.task('fonts', function() {
	return gulp.src(SRC + 'fonts/**/*.{ttf,woff,woff2,eof,svg}')
		.pipe(gulp.dest(DST + 'fonts/'));
});

gulp.task('favicon', function() {
	return gulp.src(SRC + 'favicon.ico')
		.pipe(gulp.dest(DST));
});

gulp.task('htaccess', function() {
	return gulp.src(SRC + '.htaccess')
		.pipe(gulp.dest(DST));
});

gulp.task('robots', function() {
	return gulp.src(SRC + 'robots.txt')
		.pipe(gulp.dest(DST));
});

gulp.task('sitemap', function() {
	return gulp.src(SRC + 'sitemap.xml')
		.pipe(gulp.dest(DST));
});

gulp.task('htmlkit', function() {
	return gulp.src(SRC + '*.kit')
		.pipe(kit({
			compilePartials: true
		}))
		.pipe(gulp.dest(DST + ''));
});

gulp.task('htmlmin', function() {
	return gulp.src(DST + '*.html')
		.pipe(htmlmin({
			collapseWhitespace: true,
			removeComments: true
		}))
		.pipe(gulp.dest(DST));
});

gulp.task('sass', function() {
	return merge(
			gulp.src(SRC + 'css/estilos.scss')
			.pipe(sass())
			.pipe(uncss({
				html: [DST + '*.html']
			})),

			gulp.src(SRC + 'css/looper.scss')
			.pipe(sass()),

			gulp.src(SRC + 'css/tooltip.scss')
			.pipe(sass()),

			gulp.src(SRC + 'css/acordeon.scss')
			.pipe(sass())

		)
		.pipe(autoprefixer())
		.pipe(concat('estilos.css'))
		.pipe(nano())
		.pipe(gulp.dest(DST + 'css/'));
});



// gulp.task('default', ['browser-sync'], function() {
// 	gulp.watch("src/css/**/*.scss", ['styles']);
// 	gulp.watch("src/js/**/*.js", ['scripts']);
// 	gulp.watch("*.html", ['bs-reload']);
// });