Definetely plants ???

1 what is the current estimate on how many different specie of plants.??

Myself I never counted a quick look at Bold's .. "The Plant Kingdom" states approximately ...

... flowering plants around 250,000 .. Conifers 550 .. Ferns 9,500 ... Mosses 12,000 ... not including Fungi and bacteria organisms about 300,000 plants more or less ...

2 I've heard estimates that 3/4 of all plant specie occur only within the tropics and there are probably at least an equal amount of unknown specie??

There is great diversity found in the tropics ... that is the number of species per acre as is true of other organisms there as well ... and there are many unknown plants there as well ... not sure off hand about world wide distribution.

3 What is the most common plant?? Again just those that a gardener would identify as a plant not those that can't make up their mind lol.

Indeed it is the age of the flowering plants .... what is most common gardening plant ... I don't know.

4 What is the current estimate on natural hybrids??

Beats me ... too many for me to count ?????
5 Does grafting happen naturally??

Yes it can ... root grafting as well as branches.

Check out : "The Grafters Handbook" by RJ Garner.