What specialization shall I choose?

That depends on what type of work you are really interested in.

Since you are not good in Finance, don't pursue that one. All of the choices involve strong math skills. Marketing Management involves lots of sales, media communication, personal contacts, etc. Strategy Management involves lots of planning. Operations, Quality, and Supply Change Management involves planning and managing resources, needs lots of good people skills. If you plan to continue working for an industrial firm, Operations may be the one best suited for your interests. Operations also translates well across many industries.

Here is a free website that you may find of interest in your decision making process:

http://www.careertest.net/

It helps identify the type of work you are best suited for. It is a set of 60 questions, takes about 10 minutes to answer. The results may surprise you, or confirm your opinion.

Here are some career definitions for MBA concentrations from Wharton. They should help you define your area of interest:

Finance:

Wharton's most popular major, Finance is also a common choice for a double major. It clearly appeals to those interested in the finance sector, including investment banking, investment management, hedge funds, and private equity. The major is likewise a good choice for those considering careers in corporate treasury, retail banking, and consumer finance positions. Finance majors also often fill management consulting jobs as well as corporate finance positions, for example in mergers and acquisitions in the energy sector. The Finance major combines particularly well with Strategic Management, Marketing, and Operations and Information Management.

Strategic Management:

The Strategic Management major is relevant for those pursuing a management consulting career. It is also a popular choice for those going into general management or internal consulting. Strategic Management also combines well with the Marketing major for those interested in product and brand management or the Finance major for those going into management consulting.

Marketing:

The Marketing major investigates the fundamental approaches to understanding markets, competitors, and portfolios of product offerings. It is an excellent choice for those pursuing general management careers because it examines the fundamental approaches to understanding markets, competitors, and portfolios of product offerings. It is popular for those interested in the media and entertainment field, as well as in management consulting. Marketing's flexibility makes it a good choice for a double major with Finance, Strategic Management, Entrepreneurial Management, or Operations Management.

Operations and Information Management:

The Operations and Information Management major is relevant for those involved in the "supply chain" of a business, whether it be with a physical supply chain or an information-based system within the retail or manufacturing end. The automotive and transportation industries are good examples of increasingly complex business areas that clearly benefit from the skills developed in this major. Individuals studying Operations and Information Management will also find attractive opportunities in management consulting and private equity firms.

And a combo major offering:

Marketing and Operations Management (joint major)

Comprising seven courses, Marketing and Operations Management is the only official "joint major" at Wharton -- although many students pursue double, and sometimes triple, majors. This area is attractive to those interested in marketing in the manufacturing and service sectors (business-to-business marketing in general) (e.g. supply chain management and new product development, and general product and brand management).