What is the best modification suggestion for pulsar 150 dtsi 2005 model

It depends. You can alter your bike according to the budget.
Exhaust is the common stuff everyone uses to change. It announces a throaty arrival of the bike.
Then the headlight. Attaching a small headlight that is aggressively designed will give a sporty appearance.
Wear off the fully covered chain cover and replace it with a minimal-opened chain cover.
Adding split seat (with a raised pillion seat) will add impressiveness to the machine.
If you could spend more, change the rear light with a sporty one. Along with it try to replace the rear mudguard.
You can also change the positions of both no. plates.
Painting the bike in matte black color or in any other flashy shade along with a matching alloy wheel shade would be even sportier. Adding graphics to fuel tank and other body parts around the seat would be great. You can also think about another alloy wheel with different no. of spokes and all.
Then the another important stuff is the handlebar. Putting in either a short or a wide handle bar can welcome aggressiveness to the bike's total ambiance. And remember to fix a couple of sporty rear view mirrors as well.
These are pretty expensive. So do according to your budget.
Have a nice day.
Than you.