Shared hosting (i.e. just buying slots)? The cheapest and the easiest. A VPS or dedi requires a thorough knowledge of linux and its applications, which I know most beginners don't have.
Being the most expensive does not equal being the most professional. Many servers run their forums on phpBB or SMF and they are doing just fine. With a good skin you can make anything look professional.

Besides that, don't use nulled software, ever. If I see a a vB or IPB on a webhost while the owner can barely pay for the game server itself then something's fishy and I WILL report that site to Internet Brands/Invision Power. Following an official complaint from either of those your host will at the very least disable your website, but they might as well terminate the entire contract. Obviously without refunds.