I've got ratchets, sockets, wrenches, allen and torx bits. Most "special" tools can be substituted. A punched out allen bit and a adjustable wrench substitute for BMW's special bit, a c-clamp for their pusher, etc..

Variety helps too as I've needed multiple sockets of the same size so I either use a wrench or a similar SAE socket.

Edit: Looking at my tool chest, what SlickGT1 listed below is on the mark.

A rubber mallet and mini-sledge has come in handy. Various pieces of wood and wedges have also helped dislodge items.
Extensions and flex joints are a must to get into tight spaces, such as work for thrust arms
Torque wrenches are a given.
Plastic pry tools have paid for themselves over and over working on the interior.
I have a stubby tool set with a screwdriver with interchangeable heads and ratchet that I use for interior work.
A recent addition is a LED headlamp as you can't always get light underneath the car.
A good supply of chemicals and greases helps too. Brake cleaner, PB Blaster, WD40, brake grease, anti-seize and just synthetic lube has been quite useful.
Paper towels and work gloves to keep yourself from getting too dirty. Rubber gloves when working with greases.

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