All the above listed holiday destinations are very popular and attructive for travellers, i already explored most of tge destinations but in my opinion Gangtok and Darjeeling would be the best for natural beauty and snow covered peaks and a spectacular view of mount Kanchenjunga.
Darjeeling belongs to West Bengal states very nearest to my home city Kolkata and very fameous for Tea garden and Toy Train. It is fameous from British Era. Kalimpong, Mirik, Lava, Lolegaon, Rishop, Lepchajagat are also attruction near Darjeeling.
Gangtok is the capital of Sikkim a city which attract travellers to visit many times for its natual beauty.
Sikkim has several tourist destinations divided into four districts such as North Sikkim consist of Yumthang Valley, Gurudongmar Lake, Lachen and Lachung. Gangtok city is located in eastern part of sikkim along with Tsombo Lake, Nathula Pass (Indo-China border), Baba Harbhajan Singh Mandir, Zuluk, Kupup, Aritar, Nathang valley etc. West Sikkim is also known as Pelling, Yuksom was the old capital of sikkim belongs to west sikkim along with Rinchenpong, Uttorey, Versey etc.
South Sikkim is connecting Sikkim with main Indian lands in Siliguri corridor Ravangla, Namchi, Chardham thise are the main attraction of southern sikkim.
Overall it is very much enjoyable holiday destinations for 7-8 days within budget. Nearest airport is Bagdogra and Railway Station is New Jalpaiguri known as NJP.