As far as India is concerned, the biggest fish is CAT
Give or take 90 out of the top 100 select based on CAT, with quite of a few of these selecting based only on CAT. This includes the IIMs (13 of these, soon to be 18), IITs, SP Jain, FMS, Bajaj, NITIE, MDI, IMT, among a great many others
For international MBAs, it is GMAT
Almost all universities in the US, UK, Australia, Singapore, Europe select based on GMAT. In India ISB and Great Lakes select based on GMAT. All the IIMs (+ MDI, XLRI etc) use the GMAT score for admissions into their executive MBA programs.
CAT or GMAT, which one should I take?
There are two simple rules
1. If you want to do in India, take CAT. If you want to go abroad, take GMAT.
2. If you are a fresher, take CAT. If you have more than 8 years of experience, give preference to GMAT
If you have 2-4 years of experience, and not sure whether you want to do MBA in India or abroad, then the decision is even simpler. Take both.
What about XAT?
There are around 30 decent colleges that select based on XAT. Within this, probably XLRI is the only college that selects only through XAT. Almost everyone else accepts CAT also (XIMB, LIBA GIM etc)
The big two are CAT and GMAT really, XAT is important because it can give you a second option.
What about these other exams?
Now, IIFT selects based on a separate exam. Symbiosis uses SNAP, NMIMS has NMAT. These three give you access to narrow groups of colleges, so write these based on whether you like that particular college. There is a government-mandated CMAT and then MAT. Of these, CMAT is more or less useless. MAT is accepted in a lot of private colleges, but there is practically no college that accepts MAT that will not give you a seat based on a good CAT score. If you are confident of getting 90th percentile in CAT, then skip MAT.
What about the quality of the exam?
CAT is of a good quality (note high quality, not difficulty). The quality of the GMAT exam is even better. Now, what do I mean by quality? The questions are challenging but not impossible, the level of difficulty is consistent, the framework is well defined and the exam is generally well conducted. Among the others, IIFT and NMAT are alright, at least they care enough to set decent papers.
The really worst papers are XAT and SNAP. XAT papers usually carry errors and have wild variances in difficulty across sections. SNAP also do not care much about the way they set their papers. Symbiosis probably thinks it is a good idea to sell some applications and create some excitement around the college name. SNAP has been accused of even picking some questions from freely available sources on the web.
The following chart should give an idea about all these exams, their level of difficulty, their quality (subjective opinion, bear this in mind) and how many people take these exams.
Note that the bubble size broadly outlines how many prepare for these exams
So, how does one prepare for all of these?
Prepare for CAT, forget all other Indian exams. If you are taking any of the other Indian exams, all you will need to do is make are a few some adjustments that shouldn't take more than 3-4 days. Focus slightly more on XAT; because even though the paper is inconsistent XAT can open more doors than most of the other exams.
Plan for GMAT separately. Since it is an American exam, you will need to make some adjustments regarding what is more vital. GMAT tests concepts better, and de-emphasizes computation. GMAT verbal is also tougher than that of CAT. So, even though more than 75% of the syllabus is common across CAT and GMAT, a few adjustments need to be made.
How CAT, XAT and GMAT are different, syllabus-wise.
Explaining the abbreviations used in the diagram
FIJ - Fact, Inference Judgement. Even CAT Seems to be phasing this out. You should see a maximum of 1-2 questions from this in the exam.
SE - Sentence Elimination
LR - Logical Reasoning (puzzle)
WU - Word Usage
TC - Text Completion (Fill in the blanks)
PC - Paragraph Completion
DI - Data Interpretation
Trig - Trigonometry
BDM - Business Decision Making
RC - Reading Comprehension
AWA - Analytical Writing Assessment
IR - Integrated Reasoning
CR - Critical Reasoning
SC - Sentence Correction
GK - General Knowledge
P&C - Permutation and Combination.
Note that this is a broadly indicative chart. Kindly do more research on your own after keeping this as a starting point. Dont get all officious about it if I have missed some topic or category.