Hi,
There are a few things that you need to keep in mind before taking a decision.
1.How prepared are you?
2.Have you taken and diagnostic test to evaluate your current score?
3.would you want to compromise on quality for cost? (i have no idea how good/bad IMS tests are)
now if you want an opinion then here it is
1.Manhattan:Hardest quant-normal verbal (if you are scoring above 700 then go for this if not then consider strengthening your weaknesses before investing in this.)
2.Veritas-very very close to actual GMAT( alot of people have got very similar score to what they were getting in these tests.)
3.Kaplan-good
4.GMATCLUBTest-Hardest quants-highest number of tests.
PS : Manhattan, veritas and
GMATCLUB tests can be accessed free if you have sufficient redeemable points on GMATCLUB.