I am reviewing a few android tablets online and I would like to know which hardware spec (for example, CPU speed, RAM capacity etc) has the most significant impact on the smoothness (aka fluidness) of the tab..
It's not that simple. The 'headline' specs certainly matter, but so do a lot of things they don't print on the spec sheet: the RAM controller speed, the CPU and GPU L2 cache sizes, the power management settings, etc. It might have a 2 GHz quad-core CPU, but that's no use if it's always underclocked for power or heat reasons, or it's always waiting for the RAM. Cheap Chinese devices in particular often have good specs on the spec sheet, but they pair those with the cheapest components they can find, to keep the price down.
The quality of the software integration is a big factor. Old, buggy drivers, or wrong settings chosen by the manufacturer's software people, can really reduce performance. If they've also changed the system apps (particularly the launcher or the system UI), or included some always-running crapware or spyware, that'll slow things down too.
Really, the number on the product description that's best correlated with smooth performance is the one after the dollar sign.
The most impact on smoothness improvements had newer Android releases, notably 4.1 Jelly Bean, but 4.2 and 4.3 also added new techniques to make it more smooth (Project Butter in 4.1, triple buffering in 4.3, eMMC TRIM support in 4.3).
Almost all recent hardware has 1GB+ RAM, a sufficient graphics processor and multiple cores to enable smooth UI operation.
Usually, the premium line is best, but in general look at devices with recent Android versions. Google's Nexus line has the best support here.