Since you haven't specified what style of bike you want, nor given some details about your physique, recommending something concrete will not be possible. However, we'll give you a variety of options to choose from, spanning multiple classes. 

In the neo-retro space, you can look at bikes like the Royal Enfield Classic / Bullet / Hunter 350, TVS Ronin, Triumph Speed T4 and Honda CB350 models. These will have a relaxed riding experience and a plush, comfortable ride quality. The Speed T4 will be the quickest and most responsive of them all, but still very manageable for daily use.

If you want something sportier, the KTM 250 Duke makes for a compelling option for your one-bike garage. 

If you're leaning towards the more adventurous side of things, then the Hero Xpulse 210 and the TVS Apache RTX 300 are the way to go. 

It would be advisable to take test rides of all of these bikes to see what style you prefer and which bike you are most comfortable with. 

The Honda SP160 is based on the same platform as the popular Unicorn and should have similiar strengths like good comfort, exceptional fuel efficiency, as well as durability and reliability in the long run. 

The TVS Apache RTR 160 4V is one of the best options in the sporty commuter bike segment and will offer you good performance and features without sacrificing comfort and practicality. Do bear in mind that if you are a tall person, then this bike will feel a little cramped, especially with its tight seat-to-footpeg distance.

Coming to the TVS Ronin, this bike will offer you more performance and comfort than the other 150/160cc options you're looking at, but it will return lower fuel economy. And since you're looking at the base variant, you'll only get single-channel ABS like the smaller bikes. 

The Yamaha FZ-X Hybrid will likely be the most fuel-efficient machine of this lot, but it offers lacklustre performance, and its comfort levels aren't that high either.

Our advice would be to take a test ride of everything you shortlist before making a decision. 

If you want to stay within the Royal Enfield fold, you can look at the Hunter 350, which will feel much more manageable and comfortable than your older Bullet. But if you want to get a new version of your bike, the base Battalion Black and Military variants of the Bullet 350 will fit in your budget.

The latest Bullet is a refined, comfortable and reliable machine which should return decent fuel economy, when ridden sensibly. Do bear in mind that with the Hunter, you will get dual-channel ABS. But the Bullet's base models only come with single-channel ABS and a rear drum brake. 

Alternatively, other options from reputed brands worth considering are the TVS Ronin, Apache RTR 200 4V, Honda CB300F, and Bajaj Pulsar N250. These smaller bikes will offer similar performance to the RE 350s - if not more - while being more fuel efficient too. 

Take a test ride of these machines before you make a decision. 

Within your budget, the best option would be the TVS Ronin. Compared to other comparable machines, the Ronin is a good deal lighter and has a low 795mm seat height, too. With its neutral rider's triangle, comfortable suspension setup and smooth, torque-rich engine, the Ronin will be good for both short city rides and long highway trips. 

We'd recommend getting a variant with dual-channel ABS since your budget will be able to accommodate it. 

Within your budget, you should look at bikes like the TVS Ronin, Royal Enfield Hunter 350, Honda CB350 and Triumph Speed 400. Of these four, the Speed 400 will be the fastest, but it also offers a good level of comfort and usability for daily use. Its tractable engine, short gearing and light clutch will make it a good city companion. 

The other three bikes are more relaxed in their nature but will still offer a tangible step up from your old Hornet 160, especially in the comfort they offer you. 

Take a test ride of these bikes to see which one suits your riding style best before making a decision. 

Between the TVS Ronin and the Honda Unicorn, the latter is likely to be more fuel efficient owing to its smaller engine. Both bikes are quite comfortable, and which one you find more comfortable would be dependant on your physique. 

In terms of long-term reliability, the Unicorn is an established name that has cemented its reputation for being a trouble-free workhorse. The Ronin, while much newer, also hasn't had any troubling issues reported by and large by owners. 

These are two very different bikes that you're considering, and we'd advise a test ride of both before you make a decision. 

Both bikes you have considered are good for a beginner rider. The Honda CB350 H'ness will feel like a more spacious and larger machine than the Hunter, which will be a good thing if you are a larger rider. 

On the flip side, with its smaller size and 17-inch wheels, the Royal Enfield Hunter 350 will feel more agile and flickable in city traffic. Do bear in mind that coming from an Activa, both these bikes will feel quite heavy, weighing 181kg.

Something that will be similar to a ride but substantially lighter will be the TVS Ronin, which weighs 160kg. We recommend you take a look at the Ronin instead of the two 350s. Even though you are familiar with how to operate a clutch and change gears in a car, on a motorcycle, it is a different ballgame. 

Do take a test ride of the machines you shortlist before you decide. 

The Harley-Davidson X440 will be a good starting point for a novice rider, although you will take some time to get used to its weight. Since you are shopping in this class of bike, other options to consider are Royal Enfield Hunter 350, Honda CB350 trio and the TVS Ronin. Of these, the Ronin will be the lightest and one of the most affordable options while still delivering comparable performance to the 350s. 

The Yamaha Aerox 155 is a good scooter in many ways, but its stiff rear suspension and cramped ergonomics for taller riders are two big reasons we would steer you away from it. If you do want a large-capacity scooter, then the TVS Ntorq 150 is our current pick because it has engaging performance but doesn't sacrifice comfort or practicality. 

Take a test ride of everything you consider, which will help you decide if you're ready to get into bikes or should stick with a scooter.