It just depends how much power you want and what you want out of the car.

For the cheapest swap you should stick with a GT25 or GT28 type ball bearing turbo. If you've got money to burn you could go T3/T4 but you'll have to buy a new manifold and all the plumbing will be different.

An S15 T28 can get you just above the 300 hp mark with very little lag. The next step up is the GT28 RS, it's just slightly better than the S15, it has very little lag and can get you up to 350 hp. Next is the GT2871R, it can get you up to around 400 hp with a little more lag than previous. Next would be the GT25R that should be able to flow over 400 hp if you tune it properly and upgrade other parts of the engine.

T3/T04 style turbos can be had in any A/R just about, so just pick one that flows about what you want. None of them have too much lag either unless you go really big.

It's all about what you want.